JPA: unidirectional many-to-one and cascading delete

Question

Say I have a unidirectional @ManyToOne relationship like the following:

@Entity public class Parent implements Serializable {      @Id     @GeneratedValue     private long id; }  @Entity public class Child implements Serializable {      @Id     @GeneratedValue     private long id;      @ManyToOne     @JoinColumn     private Parent parent;   } 

If I have a parent P and children C₁...C_n referencing back to P, is there a clean and pretty way in JPA to automatically remove the children C₁...C_n when P is removed (i.e. entityManager.remove(P))?

What I'm looking for is a functionality similar to ON DELETE CASCADE in SQL.

Answer 1

If you are using hibernate as your JPA provider you can use the annotation @OnDelete. This annotation will add to the relation the trigger ON DELETE CASCADE, which delegates the deletion of the children to the database.

Example:

public class Parent {             @Id         private long id;  }   public class Child {                  @Id         private long id;            @ManyToOne         @OnDelete(action = OnDeleteAction.CASCADE)         private Parent parent; }       

With this solution a unidirectional relationship from the child to the parent is enough to automatically remove all children. This solution does not need any listeners etc. Also a JPQL query like DELETE FROM Parent WHERE id = 1 will remove the children.

Answer 2

Relationships in JPA are always unidirectional, unless you associate the parent with the child in both directions. Cascading REMOVE operations from the parent to the child will require a relation from the parent to the child (not just the opposite).

You'll therefore need to do this:

• Either, change the unidirectional @ManyToOne relationship to a bi-directional @ManyToOne, or a unidirectional @OneToMany. You can then cascade REMOVE operations so that EntityManager.remove will remove the parent and the children. You can also specify orphanRemoval as true, to delete any orphaned children when the child entity in the parent collection is set to null, i.e. remove the child when it is not present in any parent's collection.
• Or, specify the foreign key constraint in the child table as ON DELETE CASCADE. You'll need to invoke EntityManager.clear() after calling EntityManager.remove(parent) as the persistence context needs to be refreshed - the child entities are not supposed to exist in the persistence context after they've been deleted in the database.