Cascading

Manually persisting multiple related entities is cumbersome. Thankfully, the JPA has a means of simplifying this process.

By default an EntityManager operation will not be applied to related entities.

To propagate EntityManager operations to a related entity, include the cascade element in the relationship annotation.

• Specify an array of operations to be cascaded.

• The values, defined in the CascadeType enum, are PERSIST, MERGE, REFRESH, REMOVE, DETACH, or ALL.

@Entity
public class Rental {
// ...
  @OneToMany(cascade=CascadeType.PERSIST)
  @JoinColumn(name = "rental_id")
  private List<Payment> payments;
// ...
}

Practice Exercise

Cascade only propagates the persist, merge, etc. actions. It does not cascade the actual data changes that will be saved by a persist or merge. You still need to set data on both sides of a bi-directional link so that persists will save the correct data in each entity.

Drill

To get some experience with cascade we will create a method to add a new film to a newly discovered actor's filmography (for the sake of this example, lets assume that our application has functionality to both create a new actor and film simultaneously).

1. Add the cascade element to the @ManyToMany relationship in Actor (the association to Film).

2. Assign cascade an array, and the PERSIST and REMOVE types.

3. In ActorDAO add the following method stub public Actor createActorAndFilm(Actor actor, Film film);.

4. In ActorDAOImpl implement the createActorAndFilm method.

5. Set the film object's language to a managed entity (use em.find(Language.class, 1)). This is a NOT NULL foreign key so it has to be set before we can persist the object.

6. Add the provided film to the Actor's list of films using the addFilm method we implemented.

7. Persist the Actor entity within a transaction.

8. In ActorDAOTest use your createActorAndFilm method to create both objects.

9. Create a Film and Actor Object

   • Give the Film:
   • a title
   • a rental rate
   • a replacement cost
   • Give the Actor:
   • a first name
   • a last name

10. Print out the Actor's toString as well as the Film's toString. You can retrieve the film by using the Actor's getFilms().get(0).

11. If you want to remove these entries from your database, you can use the destroy method in ActorDAOImpl we set up earlier.

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