Use Two Joined Columns for Many to one Mapping : Many to One Mapping « JPA « Java

1. Java
2. JPA
3. Many to One Mapping

Use Two Joined Columns for Many to one Mapping

File: Professor.java


import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.JoinColumn;
import javax.persistence.JoinColumns;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;

@Entity 
@IdClass(ProfessorId.class)
public class Professor {
    @Id private String country;
    @Id
    @Column(name="EMP_ID")
    private int id;
    private String name;
    private long salary;

    @ManyToOne 
    @JoinColumns({
        @JoinColumn(name="MGR_COUNTRY", referencedColumnName="COUNTRY"),
        @JoinColumn(name="MGR_ID", referencedColumnName="EMP_ID")
    })
    private Professor manager;

    @OneToMany(mappedBy="manager")
    private Collection<Professor> directs;

    @ManyToMany 
    @JoinTable(
        name="EMP_PROJECT", 
        joinColumns={
            @JoinColumn(name="EMP_COUNTRY", referencedColumnName="COUNTRY"),
            @JoinColumn(name="EMP_ID", referencedColumnName="EMP_ID")},
        inverseJoinColumns=@JoinColumn(name="PROJECT_ID"))
    private Collection<Project> projects;

    public Professor() {
        directs = new ArrayList<Professor>();
        projects = new ArrayList<Project>();
    }
    
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }

    public long getSalary() {
        return salary;
    }

    public void setSalary(long salary) {
        this.salary = salary;
    }
    
    public Collection<Professor> getDirects() {
        return directs;
    }
    
    public Professor getManager() {
        return manager;
    }
    
    public void setManager(Professor manager) {
        this.manager = manager;
    }
    
    public Collection<Project> getProjects() {
        return projects;
    }

    public String toString() {
        return "Professor id: " + getId() + " name: " + getName() +
               " country: " + getCountry();
    }
}


File: ProfessorId.java

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;

@Embeddable
public class ProfessorId implements Serializable{
  private String country;

  @Column(name = "EMP_ID")
  private int id;

  public ProfessorId() {
  }

  public ProfessorId(String country, int id) {
    this.country = country;
    this.id = id;
  }

  public String getCountry() {
    return country;
  }

  public int getId() {
    return id;
  }

  public boolean equals(Object o) {
    return ((o instanceof ProfessorId) && country.equals(((ProfessorId) o).getCountry()) && id == ((ProfessorId) o)
        .getId());

  }

  public int hashCode() {
    return country.hashCode() + id;
  }
}


File: ProfessorService.java

import java.util.Collection;

import javax.persistence.EntityManager;
import javax.persistence.Query;

public class ProfessorService {
  protected EntityManager em;

  public ProfessorService(EntityManager em) {
    this.em = em;
  }

  public Professor createProfessor(String country, int id, String name, long salary) {
    Professor emp = new Professor();
    emp.setCountry(country);
    emp.setId(id);
    emp.setName(name);
    emp.setSalary(salary);
    em.persist(emp);

    return emp;
  }
  public void createProject(String projName) {
    Project proj = new Project();
    proj.setName(projName);
    em.persist(proj);
  }
  public void setProfessorManager(String country, int id, String mgrCountry, int mgrId) {
    Professor emp = findProfessor(country, id);
    Professor mgr = findProfessor(mgrCountry, mgrId);
    emp.setManager(mgr);
    mgr.getDirects().add(emp);
  }

  public void addProfessorProject(String country, int id, int projId) {
    Professor emp = findProfessor(country, id);
    Project proj = em.find(Project.class, projId);
    emp.getProjects().add(proj);
    proj.getProfessors().add(emp);
  }

  public Professor findProfessor(String country, int id) {
    return em.find(Professor.class, new ProfessorId(country, id));
  }

  public Collection<Professor> findAllProfessors() {
    Query query = em.createQuery("SELECT e FROM Professor e");
    return (Collection<Professor>) query.getResultList();
  }

  public Collection<Project> findAllProjects() {
    Query query = em.createQuery("SELECT p FROM Project p");
    return (Collection<Project>) query.getResultList();
  }
}


File: Project.java

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Project {
  @Id
  protected int id;

  protected String name;

  @ManyToMany(mappedBy = "projects")
  protected Collection<Professor> employees = new ArrayList<Professor>();

  public int getId() {
    return id;
  }

  public void setId(int projectNo) {
    this.id = projectNo;
  }

  public String getName() {
    return name;
  }

  public void setName(String projectName) {
    this.name = projectName;
  }

  public Collection<Professor> getProfessors() {
    return employees;
  }

  public void addProfessor(Professor employee) {
    if (!getProfessors().contains(employee)) {
      getProfessors().add(employee);
    }
    if (!employee.getProjects().contains(this)) {
      employee.getProjects().add(this);
    }
  }

  public String toString() {
    return getClass().getName().substring(getClass().getName().lastIndexOf('.') + 1) + " no: "
        + getId() + ", name: " + getName();
  }
}


File: JPAUtil.java

import java.io.Reader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

public class JPAUtil {
  Statement st;
  
  public JPAUtil() throws Exception{
    Class.forName("org.hsqldb.jdbcDriver");
    System.out.println("Driver Loaded.");
    String url = "jdbc:hsqldb:data/tutorial";

    Connection conn = DriverManager.getConnection(url, "sa", "");
    System.out.println("Got Connection.");
    st = conn.createStatement();
  }
  public void executeSQLCommand(String sql) throws Exception {
    st.executeUpdate(sql);
  }
  public void checkData(String sql) throws Exception {
    ResultSet rs = st.executeQuery(sql);
    ResultSetMetaData metadata = rs.getMetaData();

    for (int i = 0; i < metadata.getColumnCount(); i++) {
      System.out.print("\t"+ metadata.getColumnLabel(i + 1)); 
    }
    System.out.println("\n----------------------------------");

    while (rs.next()) {
      for (int i = 0; i < metadata.getColumnCount(); i++) {
        Object value = rs.getObject(i + 1);
        if (value == null) {
          System.out.print("\t       ");
        } else {
          System.out.print("\t"+value.toString().trim());
        }
      }
      System.out.println("");
    }
  }
}


File: Main.java

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class Main {
  public static void main(String[] a) throws Exception {
    JPAUtil util = new JPAUtil();

    EntityManagerFactory emf = Persistence.createEntityManagerFactory("ProfessorService");
    EntityManager em = emf.createEntityManager();
    ProfessorService service = new ProfessorService(em);

    em.getTransaction().begin();

    service.createProfessor("country", 1, "name", 100);
    service.createProfessor("mgrcountry", 2, "mgrName", 100);
    service.createProject("proj");
    service.addProfessorProject("empCountry2", 1, 1);
    service.setProfessorManager("Country", 1, "mgrCountry", 1);



    System.out.println("Professors:");
    for (Professor emp : service.findAllProfessors()) {
      System.out.println(emp + " " + emp.getProjects().size());
      System.out.println(emp + " " + emp.getManager());
    }

    System.out.println("Projects:");
    for (Project p : service.findAllProjects()) {
      System.out.print(p + " " + p.getProfessors().size());
    }

    util.checkData("select * from Professor");

    em.getTransaction().commit();
    em.close();
    emf.close();
  }
}






File: persistence.xml

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence" version="1.0">
  <persistence-unit name="JPAService" transaction-type="RESOURCE_LOCAL">
    <properties>
      <property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
      <property name="hibernate.hbm2ddl.auto" value="update"/>
      <property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
      <property name="hibernate.connection.username" value="sa"/>
      <property name="hibernate.connection.password" value=""/>
      <property name="hibernate.connection.url" value="jdbc:hsqldb:data/tutorial"/>
    </properties>
  </persistence-unit>
</persistence>

           
       

JPA-SetJoinColumns.zip( 5,338 k)

Related examples in the same category

1.	Use One Joined Column
2.	Persisting With Relationships Using GetReference
3.	One To Many And Many To One Mapping