How to implement multilevel one to many dependencies in JPA?-CodePudding

I have 3 table.

Table A has column primary key X which is a foreign key in Table B. Table B has a column primary key Y which is a foreign key in Table C.

I created entities for these tables.In Entity A I marked Entity B as OneroMany and in Entity B i marked Entity A and ManyToOne and Entity C as OneToMany and in Entity C I marked Entity B as ManyToOne

Now when I do a save I am getting exception from Entity C that that join column with Entity B is null.

Can you help me on how to configure this multilevel dependency?

I created entities for these tables.In Entity A I marked Entity B as OneroMany and in Entity B i marked Entity A and ManyToOne and Entity C as OneToMany and in Entity C I marked Entity B as ManyToOne

Now when I do a save I am getting exception from Entity C that that join column with Entity B is null.

Can you help me on how to configure this multilevel dependency?

CodePudding user response：

This seems pretty straight out of JPA tutorials:

@Entity 
class EntityA {
  @Id
  @GeneratedValue
  @Column(name="X")
  Integer id;
  @OneToMany(mappedBy = "entityA",cascade=CascadeType.PERSIST)
  List<EntityB> entityBs;
}

@Entity 
class EntityB {
  @Id
  @GeneratedValue
  @Column(name="Y")
  Integer id;

  @ManyToOne//defaults to using entityA_id as the fk column name
  EntityA entityA;

  @OneToMany(mappedBy = "entityB",cascade=CascadeType.PERSIST)
  List<EntityC> entityCs;
}

@Entity 
class EntityC {
  @Id
  @GeneratedValue
  Integer id;

  @ManyToOne
  EntityB entityB;
}

I normally don't recommend having cascade options set, but did here just to show how I'd persist such a model using only the Root EntityA in a graph:

{
  EntityA a = new EntityA();
  EntityB b1 = new EntityB();
  a.getEntityBs().add(b1);
  b1.setEntityA(a);//set both sides of this relationship!
  EntityC c1 = new EntityC();
  b1.getEntityCs().add(c1);
  c1.setEntityB(b1);//set both sides of this relationship!

  entityManager.getTransaction().begin();
  entityManager.persist(a);
  entityManager.getTransaction().commit();
}

JPA will automatically assign ID values in to A and B and set the foreign keys into C and B from those values on the commit call.