JPA - OneToMany foreign key mapping strategy

[Updated: Apr 26, 2017, Created: Apr 13, 2017]
A quick overview of unidirectional one-to-many foreign key mapping strategy
  • By default, a unidirectional one-to-many association uses an intermediate join table to store the relationship. This default mapping is also referred as 'one-to-many join table mapping strategy'.
  • In 'one-to-many foreign key mapping strategy', instead of using a join table, we use a foreign key column in the 'target' table. This foreign key column stores the references of primary keys from the 'source' table.
  • To use this strategy we have to use @JoinColumn annotation along with @OneToMany in the source entity.

Example

@Entity
public class EntityA {
  @Id
  @GeneratedValue
  private int myIdA;
  private String strA;
  @OneToMany
  @JoinColumn(name = "MY_FK_COL")
  private List<EntityB> entityBList;
    .............
}
@Entity
public class EntityB {
  @Id
  @GeneratedValue
  private int myIdB;
  private String strB;
    .............
}
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) {
          if (o instanceof Object[]) {
              System.out.println(Arrays.toString((Object[]) o));
          } else {
              System.out.println(o);
          }
      }
  }
}

Output

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

H2 database SHOW statements

The above output shows that no 'join table' is generated, instead a foreign-key column, MY_FK_COL has been used to persist the relationship. If we don't specify 'name' element of @JoinColumn, the default ENTITYBLIST_MYIDA will be used.

Following diagram gives a quick overview of the relationship

Let's persist some data.

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) {
      System.out.println("-- native queries --");
      EntityManager em = emf.createEntityManager();
      ExampleMain.nativeQuery(em, "Select * from EntityA");
      ExampleMain.nativeQuery(em, "Select * from EntityB");
  }

  private static void persistEntity(EntityManagerFactory emf) {
      EntityManager em = emf.createEntityManager();

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

      EntityB entityB2 = new EntityB();
      entityB2.setStrB("testStringB2");

      EntityA entityA = new EntityA();
      entityA.setStrA("testStringA");
      entityA.setEntityBList(Arrays.asList(entityB, entityB2));

      System.out.println("-- Persisting entities --");
      System.out.println(entityA);

      em.getTransaction().begin();
      em.persist(entityA);
      em.persist(entityB);
      em.persist(entityB2);
      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 --
EntityA{myIdA=0, entityBList=[EntityB{myIdB=0, strB='testStringB'}, EntityB{myIdB=0, strB='testStringB2'}], strA='testStringA'}
-- native queries --
--------
'Select * from EntityA'
[1, testStringA]
--------
'Select * from EntityB'
[2, testStringB, 1]
[3, testStringB2, 1]
-- Loading EntityA --
EntityA{myIdA=1, entityBList=[EntityB{myIdB=2, strB='testStringB'}, EntityB{myIdB=3, strB='testStringB2'}], strA='testStringA'}
-- Loading EntityB --
EntityB{myIdB=2, strB='testStringB'}
EntityB{myIdB=3, strB='testStringB2'}

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

Foreign Key Mapping In One To Many Example Select All Download
  • jpa-one-to-many-foreign-key-mapping
    • src
      • main
        • java
          • com
            • logicbig
              • example
        • resources
          • META-INF

See Also