de.thm.arsnova.dao.CouchDBDao.java Source code

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/*
 * This file is part of ARSnova Backend.
 * Copyright (C) 2012-2015 The ARSnova Team
 *
 * ARSnova Backend is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ARSnova Backend is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package de.thm.arsnova.dao;

import java.io.IOException;
import java.util.AbstractMap;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;

import net.sf.ezmorph.Morpher;
import net.sf.ezmorph.MorpherRegistry;
import net.sf.ezmorph.bean.BeanMorpher;
import net.sf.json.JSONArray;
import net.sf.json.JSONObject;
import net.sf.json.util.JSONUtils;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.cache.annotation.CacheEvict;
import org.springframework.cache.annotation.CachePut;
import org.springframework.cache.annotation.Cacheable;
import org.springframework.cache.annotation.Caching;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;
import org.springframework.transaction.annotation.Isolation;
import org.springframework.transaction.annotation.Transactional;

import com.fourspaces.couchdb.Database;
import com.fourspaces.couchdb.Document;
import com.fourspaces.couchdb.Results;
import com.fourspaces.couchdb.RowResult;
import com.fourspaces.couchdb.View;
import com.fourspaces.couchdb.ViewResults;

import de.thm.arsnova.connector.model.Course;
import de.thm.arsnova.domain.CourseScore;
import de.thm.arsnova.entities.Answer;
import de.thm.arsnova.entities.DbUser;
import de.thm.arsnova.entities.InterposedQuestion;
import de.thm.arsnova.entities.InterposedReadingCount;
import de.thm.arsnova.entities.LoggedIn;
import de.thm.arsnova.entities.PossibleAnswer;
import de.thm.arsnova.entities.Question;
import de.thm.arsnova.entities.Session;
import de.thm.arsnova.entities.SessionInfo;
import de.thm.arsnova.entities.Statistics;
import de.thm.arsnova.entities.User;
import de.thm.arsnova.entities.VisitedSession;
import de.thm.arsnova.entities.transport.AnswerQueueElement;
import de.thm.arsnova.entities.transport.ImportExportSession;
import de.thm.arsnova.entities.transport.ImportExportSession.ImportExportQuestion;
import de.thm.arsnova.events.NewAnswerEvent;
import de.thm.arsnova.exceptions.NotFoundException;
import de.thm.arsnova.services.ISessionService;

@Component("databaseDao")
public class CouchDBDao implements IDatabaseDao, ApplicationEventPublisherAware {

    @Autowired
    private ISessionService sessionService;

    private String databaseHost;
    private int databasePort;
    private String databaseName;
    private Database database;

    private ApplicationEventPublisher publisher;

    private final Queue<AbstractMap.SimpleEntry<Document, AnswerQueueElement>> answerQueue = new ConcurrentLinkedQueue<AbstractMap.SimpleEntry<Document, AnswerQueueElement>>();

    public static final Logger LOGGER = LoggerFactory.getLogger(CouchDBDao.class);

    @Value("${couchdb.host}")
    public void setDatabaseHost(final String newDatabaseHost) {
        databaseHost = newDatabaseHost;
    }

    @Value("${couchdb.port}")
    public void setDatabasePort(final String newDatabasePort) {
        databasePort = Integer.parseInt(newDatabasePort);
    }

    @Value("${couchdb.name}")
    public void setDatabaseName(final String newDatabaseName) {
        databaseName = newDatabaseName;
    }

    public void setSessionService(final ISessionService service) {
        sessionService = service;
    }

    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher publisher) {
        this.publisher = publisher;
    }

    @Override
    public List<Session> getMySessions(final User user) {
        final NovaView view = new NovaView("session/by_creator");
        view.setStartKeyArray(user.getUsername());
        view.setEndKeyArray(user.getUsername(), "{}");

        final Results<Session> results = getDatabase().queryView(view, Session.class);

        final List<Session> result = new ArrayList<Session>();
        for (final RowResult<Session> row : results.getRows()) {
            final Session session = row.getValue();
            session.setCreator(row.getKey().getString(0));
            session.setName(row.getKey().getString(1));
            session.set_id(row.getId());
            result.add(session);
        }
        return result;
    }

    @Override
    public List<Session> getPublicPoolSessions() {
        final NovaView view = new NovaView("session/public_pool_by_subject");

        final ViewResults sessions = getDatabase().view(view);

        final List<Session> result = new ArrayList<Session>();

        for (final Document d : sessions.getResults()) {
            final Session session = (Session) JSONObject.toBean(d.getJSONObject().getJSONObject("value"),
                    Session.class);
            session.set_id(d.getId());
            result.add(session);
        }
        return result;
    }

    @Override
    public List<SessionInfo> getPublicPoolSessionsInfo() {
        final List<Session> sessions = this.getPublicPoolSessions();
        return getInfosForSessions(sessions);
    }

    @Override
    public List<Session> getMyPublicPoolSessions(final User user) {
        final NovaView view = new NovaView("session/public_pool_by_creator");
        view.setStartKeyArray(user.getUsername());
        view.setEndKeyArray(user.getUsername(), "{}");

        final ViewResults sessions = getDatabase().view(view);

        final List<Session> result = new ArrayList<Session>();
        for (final Document d : sessions.getResults()) {
            final Session session = (Session) JSONObject.toBean(d.getJSONObject().getJSONObject("value"),
                    Session.class);
            session.setCreator(d.getJSONObject().getJSONArray("key").getString(0));
            session.setName(d.getJSONObject().getJSONArray("key").getString(1));
            session.set_id(d.getId());
            result.add(session);
        }
        return result;
    }

    @Override
    public List<SessionInfo> getMyPublicPoolSessionsInfo(final User user) {
        final List<Session> sessions = this.getMyPublicPoolSessions(user);
        if (sessions.isEmpty()) {
            return new ArrayList<SessionInfo>();
        }
        return getInfosForSessions(sessions);
    }

    @Override
    public List<SessionInfo> getMySessionsInfo(final User user) {
        final List<Session> sessions = this.getMySessions(user);
        if (sessions.isEmpty()) {
            return new ArrayList<SessionInfo>();
        }
        return getInfosForSessions(sessions);
    }

    private List<SessionInfo> getInfosForSessions(final List<Session> sessions) {
        final ExtendedView questionCountView = new ExtendedView("skill_question/count_by_session");
        final ExtendedView answerCountView = new ExtendedView("skill_question/count_answers_by_session");
        final ExtendedView interposedCountView = new ExtendedView("interposed_question/count_by_session");
        final ExtendedView unredInterposedCountView = new ExtendedView(
                "interposed_question/count_by_session_reading");

        interposedCountView.setSessionIdKeys(sessions);
        interposedCountView.setGroup(true);
        questionCountView.setSessionIdKeys(sessions);
        questionCountView.setGroup(true);
        answerCountView.setSessionIdKeys(sessions);
        answerCountView.setGroup(true);
        List<String> unredInterposedQueryKeys = new ArrayList<String>();
        for (Session s : sessions) {
            unredInterposedQueryKeys.add("[\"" + s.get_id() + "\",\"unread\"]");
        }
        unredInterposedCountView.setKeys(unredInterposedQueryKeys);
        unredInterposedCountView.setGroup(true);
        return getSessionInfoData(sessions, questionCountView, answerCountView, interposedCountView,
                unredInterposedCountView);
    }

    private List<SessionInfo> getInfosForVisitedSessions(final List<Session> sessions, final User user) {
        final ExtendedView answeredQuestionsView = new ExtendedView("answer/by_user");
        final ExtendedView questionIdsView = new ExtendedView("skill_question/by_session_only_id_for_all");
        questionIdsView.setSessionIdKeys(sessions);
        List<String> answeredQuestionQueryKeys = new ArrayList<String>();
        for (Session s : sessions) {
            answeredQuestionQueryKeys.add("[\"" + user.getUsername() + "\",\"" + s.get_id() + "\"]");
        }
        answeredQuestionsView.setKeys(answeredQuestionQueryKeys);
        return getVisitedSessionInfoData(sessions, answeredQuestionsView, questionIdsView);
    }

    private List<SessionInfo> getVisitedSessionInfoData(List<Session> sessions, ExtendedView answeredQuestionsView,
            ExtendedView questionIdsView) {
        final Map<String, Set<String>> answeredQuestionsMap = new HashMap<String, Set<String>>();
        final Map<String, Set<String>> questionIdMap = new HashMap<String, Set<String>>();
        final ViewResults answeredQuestionsViewResults = getDatabase().view(answeredQuestionsView);
        final ViewResults questionIdsViewResults = getDatabase().view(questionIdsView);

        // Maps a session ID to a set of question IDs of answered questions of that session
        for (final Document d : answeredQuestionsViewResults.getResults()) {
            final String sessionId = d.getJSONArray("key").getString(1);
            final String questionId = d.getString("value");
            Set<String> questionIdsInSession = answeredQuestionsMap.get(sessionId);
            if (questionIdsInSession == null) {
                questionIdsInSession = new HashSet<String>();
            }
            questionIdsInSession.add(questionId);
            answeredQuestionsMap.put(sessionId, questionIdsInSession);
        }

        // Maps a session ID to a set of question IDs of that session
        for (final Document d : questionIdsViewResults.getResults()) {
            final String sessionId = d.getString("key");
            final String questionId = d.getId();
            Set<String> questionIdsInSession = questionIdMap.get(sessionId);
            if (questionIdsInSession == null) {
                questionIdsInSession = new HashSet<String>();
            }
            questionIdsInSession.add(questionId);
            questionIdMap.put(sessionId, questionIdsInSession);
        }

        // For each session, count the question IDs that are not yet answered
        Map<String, Integer> unansweredQuestionsCountMap = new HashMap<String, Integer>();
        for (final Session s : sessions) {
            if (!questionIdMap.containsKey(s.get_id())) {
                continue;
            }
            // Note: create a copy of the first set so that we don't modify the contents in the original set
            Set<String> questionIdsInSession = new HashSet<String>(questionIdMap.get(s.get_id()));
            Set<String> answeredQuestionIdsInSession = answeredQuestionsMap.get(s.get_id());
            if (answeredQuestionIdsInSession == null) {
                answeredQuestionIdsInSession = new HashSet<String>();
            }
            questionIdsInSession.removeAll(answeredQuestionIdsInSession);
            unansweredQuestionsCountMap.put(s.get_id(), questionIdsInSession.size());
        }

        List<SessionInfo> sessionInfos = new ArrayList<SessionInfo>();
        for (Session session : sessions) {
            int numUnanswered = 0;

            if (unansweredQuestionsCountMap.containsKey(session.get_id())) {
                numUnanswered = unansweredQuestionsCountMap.get(session.get_id());
            }
            SessionInfo info = new SessionInfo(session);
            info.setNumUnanswered(numUnanswered);
            sessionInfos.add(info);
        }
        return sessionInfos;
    }

    private List<SessionInfo> getSessionInfoData(final List<Session> sessions, final ExtendedView questionCountView,
            final ExtendedView answerCountView, final ExtendedView interposedCountView,
            final ExtendedView unredInterposedCountView) {
        final ViewResults questionCountViewResults = getDatabase().view(questionCountView);
        final ViewResults answerCountViewResults = getDatabase().view(answerCountView);
        final ViewResults interposedCountViewResults = getDatabase().view(interposedCountView);
        final ViewResults unredInterposedCountViewResults = getDatabase().view(unredInterposedCountView);

        Map<String, Integer> questionCountMap = new HashMap<String, Integer>();
        for (final Document d : questionCountViewResults.getResults()) {
            questionCountMap.put(d.getString("key"), d.getInt("value"));
        }
        Map<String, Integer> answerCountMap = new HashMap<String, Integer>();
        for (final Document d : answerCountViewResults.getResults()) {
            answerCountMap.put(d.getString("key"), d.getInt("value"));
        }
        Map<String, Integer> interposedCountMap = new HashMap<String, Integer>();
        for (final Document d : interposedCountViewResults.getResults()) {
            interposedCountMap.put(d.getString("key"), d.getInt("value"));
        }
        Map<String, Integer> unredInterposedCountMap = new HashMap<String, Integer>();
        for (final Document d : unredInterposedCountViewResults.getResults()) {
            unredInterposedCountMap.put(d.getJSONArray("key").getString(0), d.getInt("value"));
        }

        List<SessionInfo> sessionInfos = new ArrayList<SessionInfo>();
        for (Session session : sessions) {
            int numQuestions = 0;
            int numAnswers = 0;
            int numInterposed = 0;
            int numUnredInterposed = 0;
            if (questionCountMap.containsKey(session.get_id())) {
                numQuestions = questionCountMap.get(session.get_id());
            }
            if (answerCountMap.containsKey(session.get_id())) {
                numAnswers = answerCountMap.get(session.get_id());
            }
            if (interposedCountMap.containsKey(session.get_id())) {
                numInterposed = interposedCountMap.get(session.get_id());
            }
            if (unredInterposedCountMap.containsKey(session.get_id())) {
                numUnredInterposed = unredInterposedCountMap.get(session.get_id());
            }

            SessionInfo info = new SessionInfo(session);
            info.setNumQuestions(numQuestions);
            info.setNumAnswers(numAnswers);
            info.setNumInterposed(numInterposed);
            info.setNumUnredInterposed(numUnredInterposed);
            sessionInfos.add(info);
        }
        return sessionInfos;
    }

    /**
     * @deprecated The decision to load data depending on the user should be made by a service class, not this
     * database class. Please use getSkillQuestionsForUsers or getSkillQuestionsForTeachers as this will enable
     * caching.
     */
    @Deprecated
    @Override
    public List<Question> getSkillQuestions(final User user, final Session session) {
        String viewName;
        if (session.getCreator().equals(user.getUsername())) {
            viewName = "skill_question/by_session_sorted_by_subject_and_text";
        } else {
            viewName = "skill_question/by_session_for_all_full";
        }
        return getQuestions(new NovaView(viewName), session);
    }

    @Cacheable("skillquestions")
    @Override
    public List<Question> getSkillQuestionsForUsers(final Session session) {
        String viewName = "skill_question/by_session_for_all_full";
        return getQuestions(new NovaView(viewName), session);
    }

    @Cacheable("skillquestions")
    @Override
    public List<Question> getSkillQuestionsForTeachers(final Session session) {
        String viewName = "skill_question/by_session_sorted_by_subject_and_text";
        return getQuestions(new NovaView(viewName), session);
    }

    @Override
    public int getSkillQuestionCount(final Session session) {
        return getQuestionCount(new NovaView("skill_question/count_by_session"), session);
    }

    @Override
    @Cacheable("sessions")
    public Session getSessionFromKeyword(final String keyword) {
        final NovaView view = new NovaView("session/by_keyword");
        view.setKey(keyword);
        final ViewResults results = getDatabase().view(view);

        if (results.getJSONArray("rows").optJSONObject(0) == null) {
            throw new NotFoundException();
        }
        return (Session) JSONObject.toBean(results.getJSONArray("rows").optJSONObject(0).optJSONObject("value"),
                Session.class);
    }

    @Override
    @Cacheable("sessions")
    public Session getSessionFromId(final String sessionId) {
        final NovaView view = new NovaView("session/by_id");
        view.setKey(sessionId);

        final ViewResults sessions = getDatabase().view(view);

        if (sessions.getJSONArray("rows").optJSONObject(0) == null) {
            return null;
        }
        return (Session) JSONObject.toBean(sessions.getJSONArray("rows").optJSONObject(0).optJSONObject("value"),
                Session.class);
    }

    @Override
    public Session saveSession(final User user, final Session session) {
        final Document sessionDocument = new Document();
        sessionDocument.put("type", "session");
        sessionDocument.put("name", session.getName());
        sessionDocument.put("shortName", session.getShortName());
        sessionDocument.put("keyword", sessionService.generateKeyword());
        sessionDocument.put("creator", user.getUsername());
        sessionDocument.put("active", true);
        sessionDocument.put("courseType", session.getCourseType());
        sessionDocument.put("courseId", session.getCourseId());
        sessionDocument.put("creationTime", session.getCreationTime());
        sessionDocument.put("learningProgressType", session.getLearningProgressType());
        sessionDocument.put("ppAuthorName", session.getPpAuthorName());
        sessionDocument.put("ppAuthorMail", session.getPpAuthorMail());
        sessionDocument.put("ppUniversity", session.getPpUniversity());
        sessionDocument.put("ppLogo", session.getPpLogo());
        sessionDocument.put("ppSubject", session.getPpSubject());
        sessionDocument.put("ppLicense", session.getPpLicense());
        sessionDocument.put("ppDescription", session.getPpDescription());
        sessionDocument.put("ppFaculty", session.getPpFaculty());
        sessionDocument.put("ppLevel", session.getPpLevel());
        sessionDocument.put("sessionType", session.getSessionType());
        sessionDocument.put("features", JSONObject.fromObject(session.getFeatures()));
        try {
            database.saveDocument(sessionDocument);
        } catch (final IOException e) {
            return null;
        }
        // session caching is done by loading the created session
        return getSessionFromKeyword(sessionDocument.getString("keyword"));
    }

    @Override
    @Transactional(isolation = Isolation.READ_COMMITTED)
    public boolean sessionKeyAvailable(final String keyword) {
        final View view = new View("session/by_keyword");
        final ViewResults results = getDatabase().view(view);

        return !results.containsKey(keyword);
    }

    private String getSessionKeyword(final String internalSessionId) throws IOException {
        final Document document = getDatabase().getDocument(internalSessionId);
        if (document.has("keyword")) {
            return (String) document.get("keyword");
        }
        LOGGER.error("No session found for internal id: {}", internalSessionId);
        return null;
    }

    private Database getDatabase() {
        if (database == null) {
            try {
                final com.fourspaces.couchdb.Session session = new com.fourspaces.couchdb.Session(databaseHost,
                        databasePort);
                database = session.getDatabase(databaseName);
            } catch (final Exception e) {
                LOGGER.error("Cannot connect to CouchDB database '" + databaseName + "' on host '" + databaseHost
                        + "' using port " + databasePort);
            }
        }

        return database;
    }

    @CachePut(value = "questions", key = "#question")
    @Override
    public Question saveQuestion(final Session session, final Question question) {
        final Document q = toQuestionDocument(session, question);
        try {
            database.saveDocument(q);
            question.set_id(q.getId());
            question.set_rev(q.getRev());
            return question;
        } catch (final IOException e) {
            LOGGER.error("Could not save question {}", question);
        }
        return null;
    }

    private Document toQuestionDocument(final Session session, final Question question) {
        final Document q = new Document();
        q.put("type", "skill_question");
        q.put("questionType", question.getQuestionType());
        q.put("questionVariant", question.getQuestionVariant());
        q.put("sessionId", session.get_id());
        q.put("subject", question.getSubject());
        q.put("text", question.getText());
        q.put("active", question.isActive());
        q.put("number", 0); // TODO: This number is now unused. A clean up is necessary.
        q.put("releasedFor", question.getReleasedFor());
        q.put("possibleAnswers", question.getPossibleAnswers());
        q.put("noCorrect", question.isNoCorrect());
        q.put("piRound", question.getPiRound());
        q.put("showStatistic", question.isShowStatistic());
        q.put("showAnswer", question.isShowAnswer());
        q.put("abstention", question.isAbstention());
        q.put("image", question.getImage());
        q.put("fcImage", question.getFcImage());
        q.put("gridSize", question.getGridSize());
        q.put("offsetX", question.getOffsetX());
        q.put("offsetY", question.getOffsetY());
        q.put("zoomLvl", question.getZoomLvl());
        q.put("gridOffsetX", question.getGridOffsetX());
        q.put("gridOffsetY", question.getGridOffsetY());
        q.put("gridZoomLvl", question.getGridZoomLvl());
        q.put("gridSizeX", question.getGridSizeX());
        q.put("gridSizeY", question.getGridSizeY());
        q.put("gridIsHidden", question.getGridIsHidden());
        q.put("imgRotation", question.getImgRotation());
        q.put("toggleFieldsLeft", question.getToggleFieldsLeft());
        q.put("numClickableFields", question.getNumClickableFields());
        q.put("thresholdCorrectAnswers", question.getThresholdCorrectAnswers());
        q.put("cvIsColored", question.getCvIsColored());
        q.put("gridLineColor", question.getGridLineColor());
        q.put("numberOfDots", question.getNumberOfDots());
        q.put("gridType", question.getGridType());
        q.put("scaleFactor", question.getScaleFactor());
        q.put("gridScaleFactor", question.getGridScaleFactor());
        q.put("imageQuestion", question.isImageQuestion());
        q.put("textAnswerEnabled", question.isTextAnswerEnabled());

        return q;
    }

    @Caching(evict = { @CacheEvict(value = "skillquestions", allEntries = true),
            @CacheEvict(value = "lecturequestions", allEntries = true),
            @CacheEvict(value = "preparationquestions", allEntries = true),
            @CacheEvict(value = "flashcardquestions", allEntries = true) }, put = { @CachePut("questions") })
    @Override
    public Question updateQuestion(final Question question) {
        try {
            final Document q = database.getDocument(question.get_id());
            q.put("subject", question.getSubject());
            q.put("text", question.getText());
            q.put("active", question.isActive());
            q.put("releasedFor", question.getReleasedFor());
            q.put("possibleAnswers", question.getPossibleAnswers());
            q.put("noCorrect", question.isNoCorrect());
            q.put("piRound", question.getPiRound());
            q.put("showStatistic", question.isShowStatistic());
            q.put("showAnswer", question.isShowAnswer());
            q.put("abstention", question.isAbstention());
            q.put("image", question.getImage());
            q.put("fcImage", question.getFcImage());
            q.put("gridSize", question.getGridSize());
            q.put("offsetX", question.getOffsetX());
            q.put("offsetY", question.getOffsetY());
            q.put("zoomLvl", question.getZoomLvl());
            q.put("gridOffsetX", question.getGridOffsetX());
            q.put("gridOffsetY", question.getGridOffsetY());
            q.put("gridZoomLvl", question.getGridZoomLvl());
            q.put("gridSizeX", question.getGridSizeX());
            q.put("gridSizeY", question.getGridSizeY());
            q.put("gridIsHidden", question.getGridIsHidden());
            q.put("imgRotation", question.getImgRotation());
            q.put("toggleFieldsLeft", question.getToggleFieldsLeft());
            q.put("numClickableFields", question.getNumClickableFields());
            q.put("thresholdCorrectAnswers", question.getThresholdCorrectAnswers());
            q.put("cvIsColored", question.getCvIsColored());
            q.put("gridLineColor", question.getGridLineColor());
            q.put("numberOfDots", question.getNumberOfDots());
            q.put("gridType", question.getGridType());
            q.put("scaleFactor", question.getScaleFactor());
            q.put("gridScaleFactor", question.getGridScaleFactor());
            q.put("imageQuestion", question.isImageQuestion());

            database.saveDocument(q);
            question.set_rev(q.getRev());

            return question;
        } catch (final IOException e) {
            LOGGER.error("Could not update question {}", question);
        }

        return null;
    }

    @Override
    public InterposedQuestion saveQuestion(final Session session, final InterposedQuestion question, User user) {
        final Document q = new Document();
        q.put("type", "interposed_question");
        q.put("sessionId", session.get_id());
        q.put("subject", question.getSubject());
        q.put("text", question.getText());
        if (question.getTimestamp() != 0) {
            q.put("timestamp", question.getTimestamp());
        } else {
            q.put("timestamp", System.currentTimeMillis());
        }
        q.put("read", false);
        q.put("creator", user.getUsername());
        try {
            database.saveDocument(q);
            question.set_id(q.getId());
            question.set_rev(q.getRev());

            return question;
        } catch (final IOException e) {
            LOGGER.error("Could not save interposed question {}", question);
        }

        return null;
    }

    @Cacheable("questions")
    @Override
    public Question getQuestion(final String id) {
        try {
            final Document q = getDatabase().getDocument(id);
            final Question question = (Question) JSONObject.toBean(q.getJSONObject(), Question.class);
            final JSONArray possibleAnswers = q.getJSONObject().getJSONArray("possibleAnswers");
            @SuppressWarnings("unchecked")
            final Collection<PossibleAnswer> answers = JSONArray.toCollection(possibleAnswers,
                    PossibleAnswer.class);
            question.setPossibleAnswers(new ArrayList<PossibleAnswer>(answers));
            question.setSessionKeyword(getSessionKeyword(question.getSessionId()));
            return question;
        } catch (final IOException e) {
            LOGGER.error("Could not get question with id {}", id);
        }
        return null;
    }

    @Override
    public LoggedIn registerAsOnlineUser(final User user, final Session session) {
        try {
            final NovaView view = new NovaView("logged_in/all");
            view.setKey(user.getUsername());
            final ViewResults results = getDatabase().view(view);

            LoggedIn loggedIn = new LoggedIn();
            if (results.getJSONArray("rows").optJSONObject(0) != null) {
                final JSONObject json = results.getJSONArray("rows").optJSONObject(0).optJSONObject("value");
                loggedIn = (LoggedIn) JSONObject.toBean(json, LoggedIn.class);
                final JSONArray vs = json.optJSONArray("visitedSessions");
                if (vs != null) {
                    @SuppressWarnings("unchecked")
                    final Collection<VisitedSession> visitedSessions = JSONArray.toCollection(vs,
                            VisitedSession.class);
                    loggedIn.setVisitedSessions(new ArrayList<VisitedSession>(visitedSessions));
                }

                /* Do not clutter CouchDB. Only update once every 3 hours per session. */
                if (loggedIn.getSessionId().equals(session.get_id())
                        && loggedIn.getTimestamp() > System.currentTimeMillis() - 3 * 3600000) {
                    return loggedIn;
                }
            }

            loggedIn.setUser(user.getUsername());
            loggedIn.setSessionId(session.get_id());
            loggedIn.addVisitedSession(session);
            loggedIn.updateTimestamp();

            final JSONObject json = JSONObject.fromObject(loggedIn);
            final Document doc = new Document(json);
            if (doc.getId().isEmpty()) {
                // If this is a new user without a logged_in document, we have
                // to remove the following
                // pre-filled fields. Otherwise, CouchDB will take these empty
                // fields as genuine
                // identifiers, and will throw errors afterwards.
                doc.remove("_id");
                doc.remove("_rev");
            }
            getDatabase().saveDocument(doc);

            final LoggedIn l = (LoggedIn) JSONObject.toBean(doc.getJSONObject(), LoggedIn.class);
            final JSONArray vs = doc.getJSONObject().optJSONArray("visitedSessions");
            if (vs != null) {
                @SuppressWarnings("unchecked")
                final Collection<VisitedSession> visitedSessions = JSONArray.toCollection(vs, VisitedSession.class);
                l.setVisitedSessions(new ArrayList<VisitedSession>(visitedSessions));
            }
            return l;
        } catch (final IOException e) {
            return null;
        }
    }

    @Override
    @CachePut(value = "sessions")
    public Session updateSessionOwnerActivity(final Session session) {
        try {
            /* Do not clutter CouchDB. Only update once every 3 hours. */
            if (session.getLastOwnerActivity() > System.currentTimeMillis() - 3 * 3600000) {
                return session;
            }

            session.setLastOwnerActivity(System.currentTimeMillis());
            final JSONObject json = JSONObject.fromObject(session);
            getDatabase().saveDocument(new Document(json));
            return session;
        } catch (final IOException e) {
            LOGGER.error("Failed to update lastOwnerActivity for Session {}", session);
            return session;
        }
    }

    @Override
    public List<String> getQuestionIds(final Session session, final User user) {
        NovaView view = new NovaView("skill_question/by_session_only_id_for_all");
        view.setKey(session.get_id());
        return collectQuestionIds(view);
    }

    @Caching(evict = { @CacheEvict("questions"), @CacheEvict(value = "skillquestions", allEntries = true),
            @CacheEvict(value = "lecturequestions", allEntries = true),
            @CacheEvict(value = "preparationquestions", allEntries = true),
            @CacheEvict(value = "flashcardquestions", allEntries = true) })
    @Override
    public void deleteQuestionWithAnswers(final Question question) {
        try {
            deleteAnswers(question);
            deleteDocument(question.get_id());
        } catch (final IOException e) {
            LOGGER.error("IOException: Could not delete question {}", question.get_id());
        }
    }

    @Caching(evict = { @CacheEvict("questions"), @CacheEvict(value = "skillquestions", allEntries = true),
            @CacheEvict(value = "lecturequestions", allEntries = true),
            @CacheEvict(value = "preparationquestions", allEntries = true),
            @CacheEvict(value = "flashcardquestions", allEntries = true) })
    @Override
    public void deleteAllQuestionsWithAnswers(final Session session) {
        final NovaView view = new NovaView("skill_question/by_session");
        deleteAllQuestionDocumentsWithAnswers(session, view);
    }

    private void deleteAllQuestionDocumentsWithAnswers(final Session session, final NovaView view) {
        view.setStartKeyArray(session.get_id());
        view.setEndKey(session.get_id(), "{}");
        final ViewResults results = getDatabase().view(view);

        List<Question> questions = new ArrayList<Question>();
        for (final Document d : results.getResults()) {
            final Question q = new Question();
            q.set_id(d.getId());
            q.set_rev(d.getJSONObject("value").getString("_rev"));
            questions.add(q);
        }
        deleteAllAnswersWithQuestions(questions);
    }

    private void deleteDocument(final String documentId) throws IOException {
        final Document d = getDatabase().getDocument(documentId);
        getDatabase().deleteDocument(d);
    }

    @CacheEvict("answers")
    @Override
    public void deleteAnswers(final Question question) {
        try {
            final NovaView view = new NovaView("answer/cleanup");
            view.setKey(question.get_id());
            view.setIncludeDocs(true);
            final ViewResults results = getDatabase().view(view);

            List<Document> answersToDelete = new ArrayList<Document>();
            for (final Document a : results.getResults()) {
                final Document d = new Document(a.getJSONObject("doc"));
                d.put("_deleted", true);
                answersToDelete.add(d);
            }
            database.bulkSaveDocuments(answersToDelete.toArray(new Document[answersToDelete.size()]));
        } catch (final IOException e) {
            LOGGER.error("IOException: Could not delete answers for question {}", question.get_id());
        }
    }

    @Override
    public List<String> getUnAnsweredQuestionIds(final Session session, final User user) {
        final NovaView view = new NovaView("answer/by_user");
        view.setKey(user.getUsername(), session.get_id());
        return collectUnansweredQuestionIds(getQuestionIds(session, user), view);
    }

    @Override
    public Answer getMyAnswer(final User me, final String questionId, final int piRound) {

        final NovaView view = new NovaView("answer/by_question_and_user_and_piround");
        if (2 == piRound) {
            view.setKey(questionId, me.getUsername(), "2");
        } else {
            /* needed for legacy questions whose piRound property has not been set */
            view.setStartKey(questionId, me.getUsername());
            view.setEndKey(questionId, me.getUsername(), "1");
        }
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().isEmpty()) {
            return null;
        }
        return (Answer) JSONObject.toBean(results.getJSONArray("rows").optJSONObject(0).optJSONObject("value"),
                Answer.class);
    }

    @Override
    public List<Answer> getAnswers(final Question question, final int piRound) {
        final String questionId = question.get_id();
        final NovaView view = new NovaView("skill_question/count_answers_by_question_and_piround");
        if (2 == piRound) {
            view.setStartKey(questionId, "2");
            view.setEndKey(questionId, "2", "{}");
        } else {
            /* needed for legacy questions whose piRound property has not been set */
            view.setStartKeyArray(questionId);
            view.setEndKeyArray(questionId, "1", "{}");
        }
        view.setGroup(true);
        final ViewResults results = getDatabase().view(view);
        final int abstentionCount = getAbstentionAnswerCount(questionId);
        final List<Answer> answers = new ArrayList<Answer>();
        for (final Document d : results.getResults()) {
            final Answer a = new Answer();
            a.setAnswerCount(d.getInt("value"));
            a.setAbstentionCount(abstentionCount);
            a.setQuestionId(d.getJSONObject().getJSONArray("key").getString(0));
            a.setPiRound(piRound);
            final String answerText = d.getJSONObject().getJSONArray("key").getString(2);
            a.setAnswerText("null".equals(answerText) ? null : answerText);
            answers.add(a);
        }
        return answers;
    }

    @Cacheable("answers")
    @Override
    public List<Answer> getAnswers(final Question question) {
        return this.getAnswers(question, question.getPiRound());
    }

    @Override
    public int getAbstentionAnswerCount(final String questionId) {
        final NovaView view = new NovaView("skill_question/count_abstention_answers_by_question");
        view.setKey(questionId);
        view.setGroup(true);
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().size() == 0) {
            return 0;
        }
        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    @Override
    public int getAnswerCount(final Question question, final int piRound) {
        final NovaView view = new NovaView("skill_question/count_total_answers_by_question_and_piround");
        view.setGroup(true);
        view.setStartKey(question.get_id(), String.valueOf(piRound));
        view.setEndKey(question.get_id(), String.valueOf(piRound), "{}");
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().size() == 0) {
            return 0;
        }

        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    private boolean isEmptyResults(final ViewResults results) {
        return results == null || results.getResults().isEmpty() || results.getJSONArray("rows").size() == 0;
    }

    @Override
    public List<Answer> getFreetextAnswers(final String questionId) {
        final List<Answer> answers = new ArrayList<Answer>();
        final NovaView view = new NovaView("skill_question/freetext_answers_full");
        view.setKey(questionId);
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().isEmpty()) {
            return answers;
        }
        for (final Document d : results.getResults()) {
            final Answer a = (Answer) JSONObject.toBean(d.getJSONObject().getJSONObject("value"), Answer.class);
            a.setQuestionId(questionId);
            answers.add(a);
        }
        return answers;
    }

    @Override
    public List<Answer> getMyAnswers(final User me, final Session s) {
        final NovaView view = new NovaView("answer/by_user_and_session_full");
        view.setKey(me.getUsername(), s.get_id());
        final ViewResults results = getDatabase().view(view);
        final List<Answer> answers = new ArrayList<Answer>();
        if (results == null || results.getResults() == null || results.getResults().isEmpty()) {
            return answers;
        }
        for (final Document d : results.getResults()) {
            final Answer a = (Answer) JSONObject.toBean(d.getJSONObject().getJSONObject("value"), Answer.class);
            a.set_id(d.getId());
            a.set_rev(d.getRev());
            a.setUser(me.getUsername());
            a.setSessionId(s.get_id());
            answers.add(a);
        }
        return answers;
    }

    @Override
    public int getTotalAnswerCount(final String sessionKey) {
        final Session s = getSessionFromKeyword(sessionKey);
        if (s == null) {
            throw new NotFoundException();
        }

        final NovaView view = new NovaView("skill_question/count_answers_by_session");
        view.setKey(s.get_id());
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().size() == 0) {
            return 0;
        }
        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    @Override
    public int getInterposedCount(final String sessionKey) {
        final Session s = getSessionFromKeyword(sessionKey);
        if (s == null) {
            throw new NotFoundException();
        }

        final NovaView view = new NovaView("interposed_question/count_by_session");
        view.setKey(s.get_id());
        view.setGroup(true);
        final ViewResults results = getDatabase().view(view);
        if (results.size() == 0 || results.getResults().size() == 0) {
            return 0;
        }
        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    @Override
    public InterposedReadingCount getInterposedReadingCount(final Session session) {
        final NovaView view = new NovaView("interposed_question/count_by_session_reading");
        view.setStartKeyArray(session.get_id());
        view.setEndKeyArray(session.get_id(), "{}");
        view.setGroup(true);
        return getInterposedReadingCount(view);
    }

    @Override
    public InterposedReadingCount getInterposedReadingCount(final Session session, final User user) {
        final NovaView view = new NovaView("interposed_question/count_by_session_reading_for_creator");
        view.setStartKeyArray(session.get_id(), user.getUsername());
        view.setEndKeyArray(session.get_id(), user.getUsername(), "{}");
        view.setGroup(true);
        return getInterposedReadingCount(view);
    }

    private InterposedReadingCount getInterposedReadingCount(final NovaView view) {
        final ViewResults results = getDatabase().view(view);
        if (results.size() == 0 || results.getResults().size() == 0) {
            return new InterposedReadingCount();
        }
        // A complete result looks like this. Note that the second row is optional, and that the first one may be
        // 'unread' or 'read', i.e., results may be switched around or only one result may be present.
        // count = {"rows":[
        // {"key":["cecebabb21b096e592d81f9c1322b877","Guestc9350cf4a3","read"],"value":1},
        // {"key":["cecebabb21b096e592d81f9c1322b877","Guestc9350cf4a3","unread"],"value":1}
        // ]}
        int read = 0, unread = 0;
        String type = "";
        final JSONObject fst = results.getJSONArray("rows").getJSONObject(0);
        final JSONObject snd = results.getJSONArray("rows").optJSONObject(1);

        final JSONArray fstkey = fst.getJSONArray("key");
        if (fstkey.size() == 2) {
            type = fstkey.getString(1);
        } else if (fstkey.size() == 3) {
            type = fstkey.getString(2);
        }
        if (type.equals("read")) {
            read = fst.optInt("value");
        } else if (type.equals("unread")) {
            unread = fst.optInt("value");
        }

        if (snd != null) {
            final JSONArray sndkey = snd.getJSONArray("key");
            if (sndkey.size() == 2) {
                type = sndkey.getString(1);
            } else {
                type = sndkey.getString(2);
            }
            if (type.equals("read")) {
                read = snd.optInt("value");
            } else if (type.equals("unread")) {
                unread = snd.optInt("value");
            }
        }
        return new InterposedReadingCount(read, unread);
    }

    @Override
    public List<InterposedQuestion> getInterposedQuestions(final Session session) {
        final NovaView view = new NovaView("interposed_question/by_session_full");
        view.setKey(session.get_id());
        final ViewResults questions = getDatabase().view(view);
        if (questions == null || questions.isEmpty()) {
            return null;
        }
        return createInterposedList(session, questions);
    }

    @Override
    public List<InterposedQuestion> getInterposedQuestions(final Session session, final User user) {
        final NovaView view = new NovaView("interposed_question/by_session_and_creator");
        view.setKey(session.get_id(), user.getUsername());
        final ViewResults questions = getDatabase().view(view);
        if (questions == null || questions.isEmpty()) {
            return null;
        }
        return createInterposedList(session, questions);
    }

    private List<InterposedQuestion> createInterposedList(final Session session, final ViewResults questions) {
        final List<InterposedQuestion> result = new ArrayList<InterposedQuestion>();
        for (final Document document : questions.getResults()) {
            final InterposedQuestion question = (InterposedQuestion) JSONObject
                    .toBean(document.getJSONObject().getJSONObject("value"), InterposedQuestion.class);
            question.setSessionId(session.getKeyword());
            question.set_id(document.getId());
            result.add(question);
        }
        return result;
    }

    public InterposedQuestion getInterposedQuestion(final String sessionKey, final String documentId) {
        try {
            final Document document = getDatabase().getDocument(documentId);
            if (document == null) {
                LOGGER.error("Document is NULL");
                return null;
            }
            return (InterposedQuestion) JSONObject.toBean(document.getJSONObject(), InterposedQuestion.class);
        } catch (final IOException e) {
            LOGGER.error("Error while retrieving interposed question", e);
        }
        return null;
    }

    @Override
    public Statistics getStatistics() {
        final Statistics stats = new Statistics();
        try {
            final View statsView = new View("statistics/statistics");
            final View creatorView = new View("statistics/unique_session_creators");
            final View studentUserView = new View("statistics/active_student_users");
            statsView.setGroup(true);
            creatorView.setGroup(true);
            studentUserView.setGroup(true);

            final ViewResults statsResults = getDatabase().view(statsView);
            final ViewResults creatorResults = getDatabase().view(creatorView);
            final ViewResults studentUserResults = getDatabase().view(studentUserView);

            if (!isEmptyResults(statsResults)) {
                final JSONArray rows = statsResults.getJSONArray("rows");
                for (int i = 0; i < rows.size(); i++) {
                    final JSONObject row = rows.getJSONObject(i);
                    final int value = row.getInt("value");
                    switch (row.getString("key")) {
                    case "openSessions":
                        stats.setOpenSessions(stats.getOpenSessions() + value);
                        break;
                    case "closedSessions":
                        stats.setClosedSessions(stats.getClosedSessions() + value);
                        break;
                    case "answers":
                        stats.setAnswers(stats.getAnswers() + value);
                        break;
                    case "lectureQuestions":
                        stats.setLectureQuestions(stats.getLectureQuestions() + value);
                        break;
                    case "preparationQuestions":
                        stats.setPreparationQuestions(stats.getPreparationQuestions() + value);
                        break;
                    case "interposedQuestions":
                        stats.setInterposedQuestions(stats.getInterposedQuestions() + value);
                        break;
                    case "conceptQuestions":
                        stats.setConceptQuestions(stats.getConceptQuestions() + value);
                        break;
                    }
                }
            }
            if (!isEmptyResults(creatorResults)) {
                final JSONArray rows = creatorResults.getJSONArray("rows");
                Set<String> creators = new HashSet<String>();
                for (int i = 0; i < rows.size(); i++) {
                    final JSONObject row = rows.getJSONObject(i);
                    creators.add(row.getString("key"));
                }
                stats.setCreators(creators.size());
            }
            if (!isEmptyResults(studentUserResults)) {
                final JSONArray rows = studentUserResults.getJSONArray("rows");
                Set<String> students = new HashSet<String>();
                for (int i = 0; i < rows.size(); i++) {
                    final JSONObject row = rows.getJSONObject(i);
                    students.add(row.getString("key"));
                }
                stats.setActiveStudents(students.size());
            }
            return stats;
        } catch (final Exception e) {
            LOGGER.error("Error while retrieving session count", e);
        }
        return stats;
    }

    @Override
    public InterposedQuestion getInterposedQuestion(final String questionId) {
        try {
            final Document document = getDatabase().getDocument(questionId);
            final InterposedQuestion question = (InterposedQuestion) JSONObject.toBean(document.getJSONObject(),
                    InterposedQuestion.class);
            question.setSessionId(getSessionKeyword(question.getSessionId()));
            return question;
        } catch (final IOException e) {
            LOGGER.error("Could not load interposed question {}", questionId);
        }
        return null;
    }

    @Override
    public void markInterposedQuestionAsRead(final InterposedQuestion question) {
        try {
            question.setRead(true);
            final Document document = getDatabase().getDocument(question.get_id());
            document.put("read", question.isRead());
            getDatabase().saveDocument(document);
        } catch (final IOException e) {
            LOGGER.error("Coulg not mark interposed question as read {}", question.get_id());
        }
    }

    @Override
    public List<Session> getMyVisitedSessions(final User user) {
        final NovaView view = new NovaView("logged_in/visited_sessions_by_user");
        view.setKey(user.getUsername());
        final ViewResults sessions = getDatabase().view(view);
        final List<Session> allSessions = new ArrayList<Session>();
        for (final Document d : sessions.getResults()) {
            // Not all users have visited sessions
            if (d.getJSONObject().optJSONArray("value") != null) {
                @SuppressWarnings("unchecked")
                final Collection<Session> visitedSessions = JSONArray
                        .toCollection(d.getJSONObject().getJSONArray("value"), Session.class);
                allSessions.addAll(visitedSessions);
            }
        }
        // Filter sessions that don't exist anymore, also filter my own sessions
        final List<Session> result = new ArrayList<Session>();
        final List<Session> filteredSessions = new ArrayList<Session>();
        for (final Session s : allSessions) {
            try {
                final Session session = getSessionFromKeyword(s.getKeyword());
                if (session != null && !session.isCreator(user)) {
                    result.add(session);
                } else {
                    filteredSessions.add(s);
                }
            } catch (final NotFoundException e) {
                filteredSessions.add(s);
            }
        }
        if (filteredSessions.isEmpty()) {
            return result;
        }
        // Update document to remove sessions that don't exist anymore
        try {
            List<VisitedSession> visitedSessions = new ArrayList<VisitedSession>();
            for (final Session s : result) {
                visitedSessions.add(new VisitedSession(s));
            }
            final LoggedIn loggedIn = new LoggedIn();
            final Document loggedInDocument = getDatabase()
                    .getDocument(sessions.getResults().get(0).getString("id"));
            loggedIn.setSessionId(loggedInDocument.getString("sessionId"));
            loggedIn.setUser(user.getUsername());
            loggedIn.setTimestamp(loggedInDocument.getLong("timestamp"));
            loggedIn.setType(loggedInDocument.getString("type"));
            loggedIn.setVisitedSessions(visitedSessions);
            loggedIn.set_id(loggedInDocument.getId());
            loggedIn.set_rev(loggedInDocument.getRev());

            final JSONObject json = JSONObject.fromObject(loggedIn);
            final Document doc = new Document(json);
            getDatabase().saveDocument(doc);
        } catch (IOException e) {
            LOGGER.error("Could not clean up logged_in document of {}", user.getUsername());
        }
        return result;
    }

    @Override
    public List<SessionInfo> getMyVisitedSessionsInfo(final User user) {
        List<Session> sessions = this.getMyVisitedSessions(user);
        if (sessions.isEmpty()) {
            return new ArrayList<SessionInfo>();
        }
        return this.getInfosForVisitedSessions(sessions, user);
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public Answer saveAnswer(final Answer answer, final User user, final Question question, final Session session) {
        final Document a = new Document();
        a.put("type", "skill_question_answer");
        a.put("sessionId", answer.getSessionId());
        a.put("questionId", answer.getQuestionId());
        a.put("answerSubject", answer.getAnswerSubject());
        a.put("questionVariant", answer.getQuestionVariant());
        a.put("questionValue", answer.getQuestionValue());
        a.put("answerText", answer.getAnswerText());
        a.put("timestamp", answer.getTimestamp());
        a.put("user", user.getUsername());
        a.put("piRound", answer.getPiRound());
        a.put("abstention", answer.isAbstention());
        a.put("answerImage", answer.getAnswerImage());
        a.put("answerThumbnailImage", answer.getAnswerThumbnailImage());
        AnswerQueueElement answerQueueElement = new AnswerQueueElement(session, question, answer, user);
        this.answerQueue.offer(new AbstractMap.SimpleEntry<Document, AnswerQueueElement>(a, answerQueueElement));
        return answer;
    }

    @Scheduled(fixedDelay = 5000)
    public void flushAnswerQueue() {
        final Map<Document, Answer> map = new HashMap<Document, Answer>();
        final List<Document> answerList = new ArrayList<Document>();
        final List<AnswerQueueElement> elements = new ArrayList<AnswerQueueElement>();
        AbstractMap.SimpleEntry<Document, AnswerQueueElement> entry;
        while ((entry = this.answerQueue.poll()) != null) {
            final Document doc = entry.getKey();
            final Answer answer = entry.getValue().getAnswer();
            map.put(doc, answer);
            answerList.add(doc);
            elements.add(entry.getValue());
        }
        if (answerList.isEmpty()) {
            // no need to send an empty bulk request. ;-)
            return;
        }
        try {
            getDatabase().bulkSaveDocuments(answerList.toArray(new Document[answerList.size()]));
            for (Document d : answerList) {
                final Answer answer = map.get(d);
                answer.set_id(d.getId());
                answer.set_rev(d.getRev());
            }
            // Send NewAnswerEvents ...
            for (AnswerQueueElement e : elements) {
                this.publisher.publishEvent(
                        new NewAnswerEvent(this, e.getSession(), e.getAnswer(), e.getUser(), e.getQuestion()));
            }
        } catch (IOException e) {
            LOGGER.error("Could not bulk save answers from queue");
        }
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public Answer updateAnswer(final Answer answer) {
        try {
            final Document a = database.getDocument(answer.get_id());
            a.put("answerSubject", answer.getAnswerSubject());
            a.put("answerText", answer.getAnswerText());
            a.put("timestamp", answer.getTimestamp());
            a.put("abstention", answer.isAbstention());
            a.put("questionValue", answer.getQuestionValue());
            a.put("answerImage", answer.getAnswerImage());

            a.put("answerThumbnailImage", answer.getAnswerThumbnailImage());
            database.saveDocument(a);
            answer.set_rev(a.getRev());
            return answer;
        } catch (final IOException e) {
            LOGGER.error("Could not save answer {}", answer);
        }
        return null;
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public void deleteAnswer(final String answerId) {
        try {
            database.deleteDocument(database.getDocument(answerId));
        } catch (final IOException e) {
            LOGGER.error("Could not delete answer {} because of {}", answerId, e.getMessage());
        }
    }

    @Override
    public void deleteInterposedQuestion(final InterposedQuestion question) {
        try {
            deleteDocument(question.get_id());
        } catch (final IOException e) {
            LOGGER.error("Could not delete interposed question {} because of {}", question.get_id(),
                    e.getMessage());
        }
    }

    @Override
    public List<Session> getCourseSessions(final List<Course> courses) {
        final ExtendedView view = new ExtendedView("session/by_courseid");
        view.setCourseIdKeys(courses);

        final ViewResults sessions = getDatabase().view(view);

        final List<Session> result = new ArrayList<Session>();
        for (final Document d : sessions.getResults()) {
            final Session session = (Session) JSONObject.toBean(d.getJSONObject().getJSONObject("value"),
                    Session.class);
            result.add(session);
        }
        return result;
    }

    private static class ExtendedView extends NovaView {

        public ExtendedView(final String fullname) {
            super(fullname);
        }

        public void setCourseIdKeys(final List<Course> courses) {
            List<String> courseIds = new ArrayList<String>();
            for (Course c : courses) {
                courseIds.add(c.getId());
            }
            setKeys(courseIds);
        }

        public void setSessionIdKeys(final List<Session> sessions) {
            List<String> sessionIds = new ArrayList<String>();
            for (Session s : sessions) {
                sessionIds.add(s.get_id());
            }
            setKeys(sessionIds);
        }
    }

    @Override
    @CachePut(value = "sessions")
    public Session updateSession(final Session session) {
        try {
            final Document s = database.getDocument(session.get_id());
            s.put("name", session.getName());
            s.put("shortName", session.getShortName());
            s.put("active", session.isActive());
            s.put("learningProgressType", session.getLearningProgressType());
            s.put("features", JSONObject.fromObject(session.getFeatures()));
            database.saveDocument(s);
            session.set_rev(s.getRev());

            return session;
        } catch (final IOException e) {
            LOGGER.error("Could not lock session {}", session);
        }

        return null;
    }

    @Override
    @CacheEvict("sessions")
    public void deleteSession(final Session session) {
        try {
            deleteDocument(session.get_id());
        } catch (final IOException e) {
            LOGGER.error("Could not delete session {}", session);
        }
    }

    @Cacheable("lecturequestions")
    @Override
    public List<Question> getLectureQuestionsForUsers(final Session session) {
        String viewName = "skill_question/lecture_question_by_session_for_all";
        return getQuestions(new NovaView(viewName), session);
    }

    @Override
    public List<Question> getLectureQuestionsForTeachers(final Session session) {
        String viewName = "skill_question/lecture_question_by_session";
        return getQuestions(new NovaView(viewName), session);
    }

    @Cacheable("flashcardquestions")
    @Override
    public List<Question> getFlashcardsForUsers(final Session session) {
        String viewName = "skill_question/flashcard_by_session_for_all";
        return getQuestions(new NovaView(viewName), session);
    }

    @Override
    public List<Question> getFlashcardsForTeachers(final Session session) {
        String viewName = "skill_question/flashcard_by_session";
        return getQuestions(new NovaView(viewName), session);
    }

    @Cacheable("preparationquestions")
    @Override
    public List<Question> getPreparationQuestionsForUsers(final Session session) {
        String viewName = "skill_question/preparation_question_by_session_for_all";
        return getQuestions(new NovaView(viewName), session);
    }

    @Override
    public List<Question> getPreparationQuestionsForTeachers(final Session session) {
        String viewName = "skill_question/preparation_question_by_session";
        return getQuestions(new NovaView(viewName), session);
    }

    private List<Question> getQuestions(final NovaView view, final Session session) {
        view.setStartKeyArray(session.get_id());
        view.setEndKeyArray(session.get_id(), "{}");
        final ViewResults questions = getDatabase().view(view);
        if (questions == null || questions.isEmpty()) {
            return null;
        }
        final List<Question> result = new ArrayList<Question>();

        final MorpherRegistry morpherRegistry = JSONUtils.getMorpherRegistry();
        final Morpher dynaMorpher = new BeanMorpher(PossibleAnswer.class, morpherRegistry);
        morpherRegistry.registerMorpher(dynaMorpher);
        for (final Document document : questions.getResults()) {
            final Question question = (Question) JSONObject.toBean(document.getJSONObject().getJSONObject("value"),
                    Question.class);
            @SuppressWarnings("unchecked")
            final Collection<PossibleAnswer> answers = JSONArray.toCollection(
                    document.getJSONObject().getJSONObject("value").getJSONArray("possibleAnswers"),
                    PossibleAnswer.class);
            question.setPossibleAnswers(new ArrayList<PossibleAnswer>(answers));
            question.setSessionKeyword(session.getKeyword());
            if (!"freetext".equals(question.getQuestionType()) && 0 == question.getPiRound()) {
                /* needed for legacy questions whose piRound property has not been set */
                question.setPiRound(1);
            }
            result.add(question);
        }
        return result;
    }

    @Override
    public int getLectureQuestionCount(final Session session) {
        return getQuestionCount(new NovaView("skill_question/lecture_question_count_by_session"), session);
    }

    @Override
    public int getFlashcardCount(final Session session) {
        return getQuestionCount(new NovaView("skill_question/flashcard_count_by_session"), session);
    }

    @Override
    public int getPreparationQuestionCount(final Session session) {
        return getQuestionCount(new NovaView("skill_question/preparation_question_count_by_session"), session);
    }

    private int getQuestionCount(final NovaView view, final Session session) {
        view.setKey(session.get_id());
        final ViewResults results = getDatabase().view(view);
        if (results.getJSONArray("rows").optJSONObject(0) == null) {
            return 0;
        }
        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    @Override
    public int countLectureQuestionAnswers(final Session session) {
        return countQuestionVariantAnswers(session, "lecture");
    }

    @Override
    public int countPreparationQuestionAnswers(final Session session) {
        return countQuestionVariantAnswers(session, "preparation");
    }

    private int countQuestionVariantAnswers(final Session session, final String variant) {
        final NovaView view = new NovaView("skill_question/count_answers_by_session_and_question_variant");
        view.setKey(session.get_id(), variant);
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().size() == 0) {
            return 0;
        }
        return results.getJSONArray("rows").optJSONObject(0).optInt("value");
    }

    @Caching(evict = { @CacheEvict(value = "questions", allEntries = true),
            @CacheEvict(value = "skillquestions", allEntries = true), @CacheEvict("lecturequestions"),
            @CacheEvict(value = "answers", allEntries = true) })
    @Override
    public void deleteAllLectureQuestionsWithAnswers(final Session session) {
        final NovaView view = new NovaView("skill_question/lecture_question_by_session");
        deleteAllQuestionDocumentsWithAnswers(session, view);
    }

    @Caching(evict = { @CacheEvict(value = "questions", allEntries = true),
            @CacheEvict(value = "skillquestions", allEntries = true), @CacheEvict("flashcardquestions"),
            @CacheEvict(value = "answers", allEntries = true) })
    @Override
    public void deleteAllFlashcardsWithAnswers(final Session session) {
        final NovaView view = new NovaView("skill_question/flashcard_by_session");
        deleteAllQuestionDocumentsWithAnswers(session, view);
    }

    @Caching(evict = { @CacheEvict(value = "questions", allEntries = true),
            @CacheEvict(value = "skillquestions", allEntries = true), @CacheEvict("preparationquestions"),
            @CacheEvict(value = "answers", allEntries = true) })
    @Override
    public void deleteAllPreparationQuestionsWithAnswers(final Session session) {
        final NovaView view = new NovaView("skill_question/preparation_question_by_session");
        deleteAllQuestionDocumentsWithAnswers(session, view);
    }

    @Override
    public List<String> getUnAnsweredLectureQuestionIds(final Session session, final User user) {
        final NovaView view = new NovaView("answer/variant_by_user");
        view.setKey(user.getUsername(), session.get_id(), "lecture");
        return collectUnansweredQuestionIds(getLectureQuestionIds(session), view);
    }

    private List<String> getLectureQuestionIds(final Session session) {
        NovaView view = new NovaView("skill_question/lecture_question_ids_by_session_for_all");
        view.setStartKeyArray(session.get_id());
        view.setEndKeyArray(session.get_id(), "{}");
        return collectQuestionIds(view);
    }

    @Override
    public List<String> getUnAnsweredPreparationQuestionIds(final Session session, final User user) {
        final NovaView view = new NovaView("answer/variant_by_user");
        view.setKey(user.getUsername(), session.get_id(), "preparation");
        return collectUnansweredQuestionIds(getPreparationQuestionIds(session), view);
    }

    private List<String> getPreparationQuestionIds(final Session session) {
        NovaView view = new NovaView("skill_question/preparation_question_ids_by_session_for_all");
        view.setStartKeyArray(session.get_id());
        view.setEndKeyArray(session.get_id(), "{}");
        return collectQuestionIds(view);
    }

    private List<String> collectUnansweredQuestionIds(final List<String> questions, final NovaView view) {
        final ViewResults answeredQuestions = getDatabase().view(view);

        final List<String> answered = new ArrayList<String>();
        for (final Document d : answeredQuestions.getResults()) {
            answered.add(d.getString("value"));
        }

        final List<String> unanswered = new ArrayList<String>();
        for (final String questionId : questions) {
            if (!answered.contains(questionId)) {
                unanswered.add(questionId);
            }
        }
        return unanswered;
    }

    private List<String> collectQuestionIds(final NovaView view) {
        final ViewResults results = getDatabase().view(view);
        if (results.getResults().size() == 0) {
            return new ArrayList<String>();
        }
        final List<String> ids = new ArrayList<String>();
        for (final Document d : results.getResults()) {
            ids.add(d.getId());
        }
        return ids;
    }

    @Override
    public void deleteAllInterposedQuestions(final Session session) {
        final NovaView view = new NovaView("interposed_question/by_session");
        view.setKey(session.get_id());
        final ViewResults questions = getDatabase().view(view);
        deleteAllInterposedQuestions(session, questions);
    }

    @Override
    public void deleteAllInterposedQuestions(final Session session, final User user) {
        final NovaView view = new NovaView("interposed_question/by_session_and_creator");
        view.setKey(session.get_id(), user.getUsername());
        final ViewResults questions = getDatabase().view(view);
        deleteAllInterposedQuestions(session, questions);
    }

    private void deleteAllInterposedQuestions(final Session session, final ViewResults questions) {
        if (questions == null || questions.isEmpty()) {
            return;
        }
        for (final Document document : questions.getResults()) {
            try {
                deleteDocument(document.getId());
            } catch (final IOException e) {
                LOGGER.error("Could not delete all interposed questions {}", session);
            }
        }
    }

    @Override
    public void publishAllQuestions(final Session session, final boolean publish) {
        final List<Question> questions = getQuestions(new NovaView("skill_question/by_session"), session);
        publishQuestions(session, publish, questions);
    }

    @Caching(evict = { @CacheEvict(value = "questions", allEntries = true),
            @CacheEvict(value = "skillquestions", allEntries = true),
            @CacheEvict(value = "lecturequestions", allEntries = true),
            @CacheEvict(value = "preparationquestions", allEntries = true),
            @CacheEvict(value = "flashcardquestions", allEntries = true) })
    @Override
    public void publishQuestions(final Session session, final boolean publish, List<Question> questions) {
        for (final Question q : questions) {
            q.setActive(publish);
        }
        final List<Document> documents = new ArrayList<Document>();
        for (final Question q : questions) {
            final Document d = toQuestionDocument(session, q);
            d.setId(q.get_id());
            d.setRev(q.get_rev());
            documents.add(d);
        }
        try {
            database.bulkSaveDocuments(documents.toArray(new Document[documents.size()]));
        } catch (final IOException e) {
            LOGGER.error("Could not bulk publish all questions: {}", e.getMessage());
        }
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public void deleteAllQuestionsAnswers(final Session session) {
        final List<Question> questions = getQuestions(new NovaView("skill_question/by_session"), session);
        deleteAllAnswersForQuestions(questions);
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public void deleteAllPreparationAnswers(final Session session) {
        final List<Question> questions = getQuestions(
                new NovaView("skill_question/preparation_question_by_session"), session);
        deleteAllAnswersForQuestions(questions);
    }

    @CacheEvict(value = "answers", allEntries = true)
    @Override
    public void deleteAllLectureAnswers(final Session session) {
        final List<Question> questions = getQuestions(new NovaView("skill_question/lecture_question_by_session"),
                session);
        deleteAllAnswersForQuestions(questions);
    }

    private boolean deleteAllAnswersForQuestions(List<Question> questions) {
        List<String> questionIds = new ArrayList<String>();
        for (Question q : questions) {
            questionIds.add(q.get_id());
        }
        final NovaView bulkView = new NovaView("answer/cleanup");
        bulkView.setKeys(questionIds);
        bulkView.setIncludeDocs(true);
        final List<Document> result = getDatabase().view(bulkView).getResults();
        final List<Document> allAnswers = new ArrayList<Document>();
        for (Document a : result) {
            final Document d = new Document(a.getJSONObject("doc"));
            d.put("_deleted", true);
            allAnswers.add(d);
        }
        try {
            getDatabase().bulkSaveDocuments(allAnswers.toArray(new Document[allAnswers.size()]));
        } catch (IOException e) {
            LOGGER.error("Could not bulk delete answers: {}", e.getMessage());
            return false;
        }
        return true;
    }

    private boolean deleteAllAnswersWithQuestions(List<Question> questions) {
        List<String> questionIds = new ArrayList<String>();
        final List<Document> allQuestions = new ArrayList<Document>();
        for (Question q : questions) {
            final Document d = new Document();
            d.put("_id", q.get_id());
            d.put("_rev", q.get_rev());
            d.put("_deleted", true);
            questionIds.add(q.get_id());
            allQuestions.add(d);
        }
        final NovaView bulkView = new NovaView("answer/cleanup");
        bulkView.setKeys(questionIds);
        bulkView.setIncludeDocs(true);
        final List<Document> result = getDatabase().view(bulkView).getResults();

        final List<Document> allAnswers = new ArrayList<Document>();
        for (Document a : result) {
            final Document d = new Document(a.getJSONObject("doc"));
            d.put("_deleted", true);
            allAnswers.add(d);
        }

        try {
            List<Document> deleteList = new ArrayList<Document>(allAnswers);
            deleteList.addAll(allQuestions);
            getDatabase().bulkSaveDocuments(deleteList.toArray(new Document[deleteList.size()]));
        } catch (IOException e) {
            LOGGER.error("Could not bulk delete questions and answers: {}", e.getMessage());
            return false;
        }
        return true;
    }

    @Cacheable("learningprogress")
    @Override
    public CourseScore getLearningProgress(final Session session) {
        final NovaView maximumValueView = new NovaView("learning_progress/maximum_value_of_question");
        final NovaView answerSumView = new NovaView("learning_progress/question_value_achieved_for_user");
        maximumValueView.setStartKeyArray(session.get_id());
        maximumValueView.setEndKeyArray(session.get_id(), "{}");
        answerSumView.setStartKeyArray(session.get_id());
        answerSumView.setEndKeyArray(session.get_id(), "{}");

        final List<Document> maximumValueResult = getDatabase().view(maximumValueView).getResults();
        final List<Document> answerSumResult = getDatabase().view(answerSumView).getResults();

        CourseScore courseScore = new CourseScore();

        // no results found
        if (maximumValueResult.isEmpty() && answerSumResult.isEmpty()) {
            return courseScore;
        }

        // collect mapping (questionId -> max value)
        for (Document d : maximumValueResult) {
            String questionId = d.getJSONArray("key").getString(1);
            int questionScore = d.getInt("value");
            courseScore.add(questionId, questionScore);
        }
        // collect mapping (questionId -> (user -> value))
        for (Document d : answerSumResult) {
            String username = d.getJSONArray("key").getString(1);
            JSONObject value = d.getJSONObject("value");
            String questionId = value.getString("questionId");
            int userscore = value.getInt("score");
            courseScore.add(questionId, username, userscore);
        }
        return courseScore;
    }

    @Override
    public DbUser createOrUpdateUser(DbUser user) {
        try {
            String id = user.getId();
            String rev = user.getRev();
            Document d = new Document();

            if (null != id) {
                d = database.getDocument(id, rev);
            }

            d.put("type", "userdetails");
            d.put("username", user.getUsername());
            d.put("password", user.getPassword());
            d.put("activationKey", user.getActivationKey());
            d.put("passwordResetKey", user.getPasswordResetKey());
            d.put("passwordResetTime", user.getPasswordResetTime());
            d.put("creation", user.getCreation());
            d.put("lastLogin", user.getLastLogin());

            database.saveDocument(d, id);
            user.setId(d.getId());
            user.setRev(d.getRev());

            return user;
        } catch (IOException e) {
            LOGGER.error("Could not save user {}", user);
        }

        return null;
    }

    @Override
    public DbUser getUser(String username) {
        NovaView view = new NovaView("user/all");
        view.setKey(username);
        ViewResults results = this.getDatabase().view(view);

        if (results.getJSONArray("rows").optJSONObject(0) == null) {
            return null;
        }

        return (DbUser) JSONObject.toBean(results.getJSONArray("rows").optJSONObject(0).optJSONObject("value"),
                DbUser.class);
    }

    @Override
    public boolean deleteUser(DbUser dbUser) {
        try {
            this.deleteDocument(dbUser.getId());

            return true;
        } catch (IOException e) {
            LOGGER.error("Could not delete user {}", dbUser.getId());
        }

        return false;
    }

    @Override
    public SessionInfo importSession(User user, ImportExportSession importSession) {
        final Session session = this.saveSession(user, importSession.generateSessionEntity(user));
        List<Document> questions = new ArrayList<Document>();
        // We need to remember which answers belong to which question.
        // The answers need a questionId, so we first store the questions to get the IDs.
        // Then we update the answer objects and store them as well.
        Map<Document, ImportExportQuestion> mapping = new HashMap<Document, ImportExportQuestion>();
        // Later, generate all answer documents
        List<Document> answers = new ArrayList<Document>();
        // We can then push answers together with interposed questions in one large bulk request
        List<Document> interposedQuestions = new ArrayList<Document>();
        try {
            // add session id to all questions and generate documents
            for (ImportExportQuestion question : importSession.getQuestions()) {
                Document doc = toQuestionDocument(session, question);
                question.setSessionId(session.get_id());
                questions.add(doc);
                mapping.put(doc, question);
            }
            database.bulkSaveDocuments(questions.toArray(new Document[questions.size()]));

            // bulk import answers together with interposed questions
            for (Entry<Document, ImportExportQuestion> entry : mapping.entrySet()) {
                final Document doc = entry.getKey();
                final ImportExportQuestion question = entry.getValue();
                question.set_id(doc.getId());
                question.set_rev(doc.getRev());
                for (de.thm.arsnova.entities.transport.Answer answer : question.getAnswers()) {
                    final Answer a = answer.generateAnswerEntity(user, question);
                    final Document answerDoc = new Document();
                    answerDoc.put("type", "skill_question_answer");
                    answerDoc.put("sessionId", a.getSessionId());
                    answerDoc.put("questionId", a.getQuestionId());
                    answerDoc.put("answerSubject", a.getAnswerSubject());
                    answerDoc.put("questionVariant", a.getQuestionVariant());
                    answerDoc.put("questionValue", a.getQuestionValue());
                    answerDoc.put("answerText", a.getAnswerText());
                    answerDoc.put("timestamp", a.getTimestamp());
                    answerDoc.put("piRound", a.getPiRound());
                    answerDoc.put("abstention", a.isAbstention());
                    // we do not store the user's name
                    answerDoc.put("user", "");
                    answers.add(answerDoc);
                }
            }
            for (de.thm.arsnova.entities.transport.InterposedQuestion i : importSession.getFeedbackQuestions()) {
                final Document q = new Document();
                q.put("type", "interposed_question");
                q.put("sessionId", session.get_id());
                q.put("subject", i.getSubject());
                q.put("text", i.getText());
                q.put("timestamp", i.getTimestamp());
                q.put("read", i.isRead());
                // we do not store the creator's name
                q.put("creator", "");
                interposedQuestions.add(q);
            }
            List<Document> documents = new ArrayList<Document>(answers);
            documents.addAll(interposedQuestions);
            database.bulkSaveDocuments(documents.toArray(new Document[documents.size()]));
        } catch (IOException e) {
            LOGGER.error("Could not import this session: {}", e.getMessage());
            // Something went wrong, delete this session since we do not want a partial import
            this.deleteSession(session);
            return null;
        }
        // Calculate some statistics...
        int unreadInterposed = 0;
        for (de.thm.arsnova.entities.transport.InterposedQuestion i : importSession.getFeedbackQuestions()) {
            if (!i.isRead()) {
                unreadInterposed++;
            }
        }
        int numUnanswered = 0;
        for (ImportExportQuestion question : importSession.getQuestions()) {
            if (question.getAnswers().size() == 0) {
                numUnanswered++;
            }
        }
        final SessionInfo info = new SessionInfo(session);
        info.setNumQuestions(questions.size());
        info.setNumUnanswered(numUnanswered);
        info.setNumAnswers(answers.size());
        info.setNumInterposed(interposedQuestions.size());
        info.setNumUnredInterposed(unreadInterposed);
        return info;
    }
}