/mcls

JPA: The Difference Between Merge() and Persist()

When I started developing in Play! Framework it was my first time developing in Java and using JPA. I got a bit confused when I needed to use merge() for the first time so I decided to write a bit about it to better my understanding of Play! and JPA.

Let’s look at what the official EntityManager documentation says first.

merge()

<T> T merge(T entity)
Merge the state of the given entity into the current persistence context.

persist()

void persist(Object entity)
Make an entity instance managed and persistent.

The difference

As the documentation says persist() will make the entity instance managed and persistent, while merge() will just merge the state into the current persistence context. What this means is that both methods will add the entity to the persistence context, but persist() will also make the entity managed so that changes after the persist() call will also be persisted, whereas merge() will not do this (although it will return a managed entity instance).

An example in Play! Framework

This is where I got confused. I thought calling merge() on my model would be sufficient, but Play manages JPA a bit differently. In Play! you have to explicitly save an object using save(). Play does this because it’s more intuitive than having to call refresh() on entities. The save() method will then call persist() on the entity if needed, this is also why the models in Play don’t have a persist() method.

A very basic example (without validation etc.)

Controller

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public class Questions extends Controller {

  public static void update(Question question) {
    // Suppose the question contains some new answers
    question.merge();
    // Save! Otherwise the state of the question entity will be refreshed!
    question.save();
    render(question);
  }

}

Question Entity

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@Entity
public class Question extends Model {

  @OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)
  public List<Answer> answers;

  // Other fields...
}

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