JPA - ManyToOne relationship

[Updated: Apr 26, 2017, Created: Apr 11, 2017]
A quick overview of many-to-one relationship in JPA
  • Similar to one-to-one association, many-to-one association supports a source entity having a reference of another entity (the target entity), but unlike one-to-one association, many-to-one association also supports multiple instances of the source entity sharing a single target entity instance.
  • To establish this relationship, @ManyToOne annotation is used in the source entity class. It is placed on the filed/property of the target entity.
  • On the target side, we don't need to use any relational annotation unless it's a bidirectional association.
  • The table corresponding to the source entity, is the child-table which has a foreign key column, whereas the table corresponding to the target entity, is the parent table which has the corresponding primary key column.
  • The foreign key column in the child-table must not be unique so it can be populated with a single parent's primary key multiple times, thus forming a many to one relationship.

@ManyToOne example

@Entity
public class EntityA {
  @Id
  @GeneratedValue
  private int myIdA;
  @ManyToOne
  private EntityB refEntityB;
    .............
}
@Entity
public class EntityB {
  @Id
  @GeneratedValue
  private int myIdB;
  private String str;
    .............
}
public class ExampleMain {

  public static void main(String[] args) {
      EntityManagerFactory emf = Persistence.createEntityManagerFactory("test1");
      try {
          EntityManager em = emf.createEntityManager();
          nativeQuery(em, "SHOW TABLES");
          nativeQuery(em, "SHOW COLUMNS from EntityA");
          nativeQuery(em, "SHOW COLUMNS from EntityB");
          emf.close();
      } finally {
          emf.close();
      }
  }

  public static void nativeQuery(EntityManager em, String s) {
      System.out.printf("---------------------------%n'%s'%n", s);
      Query query = em.createNativeQuery(s);
      List list = query.getResultList();
      for (Object o : list) {
          System.out.println(Arrays.toString((Object[]) o));
      }
  }
}

Output

---------------------------
'SHOW TABLES'
[ENTITYA, PUBLIC]
[ENTITYB, PUBLIC]
---------------------------
'SHOW COLUMNS from EntityA'
[MYIDA, INTEGER(10), NO, PRI, NULL]
[REFENTITYB_MYIDB, INTEGER(10), YES, , NULL]
---------------------------
'SHOW COLUMNS from EntityB'
[MYIDB, INTEGER(10), NO, PRI, NULL]
[STR, VARCHAR(255), YES, , NULL]

H2 database SHOW statements

Here's a quick review of the relationship:

Just like @OneToOne, we can use @JoinColumn along with @ManyToOne to specify a different foreign key column name.

Persisting data

We are going to populate multiple instances of source EntityA with a single instance of target EntityB.

public class ExampleMain2 {

  public static void main(String[] args) {
      EntityManagerFactory emf = Persistence.createEntityManagerFactory("test1");
      try {
          persistEntity(emf);
          nativeQueries(emf);
          loadEntityA(emf);
          loadEntityB(emf);
      } finally {
          emf.close();
      }
  }

  private static void nativeQueries(EntityManagerFactory emf) {
      EntityManager em = emf.createEntityManager();
      ExampleMain.nativeQuery(em, "Select * from EntityA");
      ExampleMain.nativeQuery(em, "Select * from EntityB");
  }

  private static void persistEntity(EntityManagerFactory emf) {
      System.out.println("-- Persisting entities --");
      EntityManager em = emf.createEntityManager();

      EntityB entityB = new EntityB();
      entityB.setStr("testStringB");

      EntityA entityA = new EntityA();
      entityA.setRefEntityB(entityB);

      EntityA entityA2 = new EntityA();
      entityA2.setRefEntityB(entityB);


      em.getTransaction().begin();
      em.persist(entityA);
      em.persist(entityA2);
      em.persist(entityB);
      em.getTransaction().commit();

      em.close();
  }

  private static void loadEntityA(EntityManagerFactory emf) {
      System.out.println("-- Loading EntityA --");
      EntityManager em = emf.createEntityManager();
      List<EntityA> entityAList = em.createQuery("Select t from EntityA t").getResultList();
      entityAList.forEach(System.out::println);
      em.close();
  }

  private static void loadEntityB(EntityManagerFactory emf) {
      System.out.println("-- Loading EntityB --");
      EntityManager em = emf.createEntityManager();
      List<EntityB> entityBList = em.createQuery("Select t from EntityB t").getResultList();
      entityBList.forEach(System.out::println);
      em.close();
  }
}

Output

-- Persisting entities --
---------------------------
'Select * from EntityA'
[1, 3]
[2, 3]
---------------------------
'Select * from EntityB'
[3, testStringB]
-- Loading EntityA --
EntityA{myIdA=1, refEntityB=EntityB{myIdB=3, str='testStringB'}}
EntityA{myIdA=2, refEntityB=EntityB{myIdB=3, str='testStringB'}}
-- Loading EntityB --
EntityB{myIdB=3, str='testStringB'}

Example Project

Dependencies and Technologies Used :

  • h2 1.4.193: H2 Database Engine.
  • hibernate-core 5.2.8.Final: The core O/RM functionality as provided by Hibernate.
  • JDK 1.8
  • Maven 3.3.9

Many To One Relation Examples Select All Download
  • jpa-many-to-one-example
    • src
      • main
        • java
          • com
            • logicbig
              • example
        • resources
          • META-INF

See Also