What is the correct way to use CascadeType.MERGE with Spring Data Repositories?

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In this question committing the transaction that saves the instances of entity2 and entity3, which each hold a reference to an Entity1 (entity1) instance, results in two records of entity1 appearing in the database (Since the reference is saved when each of the instances are saved).

If I have 2 Spring Data Repositories - Entity2Repository and Entity3Repository and I do the following will that result in 2 instances of entity 1 being contained in the database:

Entity1 entity1 = new Entity1();
entity1.name = "Name1";
entity1.value = "Value1";

Entity2 entity2 = new Entity2();
entity2.name = "Name2";
entity2.value = "Value2";
entity2.setEntity1(entity1);

Entity3 entity3 = new Entity3();
entity3.name = "Name3";
entity3.value = "Value3";
entity3.setEntity1(entity1);

Entity2Repository.save(entity2);
Entity3Repository.save(entity3);
spring-data spring-boot jpa spring java
1 Answers

Short answer

It will work correctly (only produce one record of entity1).

Long answer

The actual result of the sample code execution depends on the @Transactional annotation presence on your test method. 

Entity2Repository.save(entity2);
Entity3Repository.save(entity3);

These calls will invoke the SimpleJpaRepository#save() method, which is @Transactional itself. 

@Transactional
public <S extends T> S save(S entity) {

    if (entityInformation.isNew(entity)) {
        em.persist(entity);
        return entity;
    } else {
        return em.merge(entity);
    }
}

Also, as you see, you should not worry about persist/merge calls - if it's a new entity, persist() will be called. isNew() check is pretty simple (see AbstractPersistable source code):

public boolean isNew() {
    return null == getId();
}

Without @Transactional

There will be two transactions - one for each save() call.

  • entity1, entity2 and entity3 are TRANSIENT now
  • Entity2Repository.save(entity2) is invoked
  • Transaction is opened, Session is created
    • persist() is invoked for entity2, as it's new entity
    • persist() is cascaded to entity1
    • entity1 and entity2 are PERSISTENT now, they are attached to the current Session
  • Transaction is commited, Session is closed
  • entity1 and entity2 are DETACHED now
  • Entity3Repository.save(entity3) is invoked
  • Transaction is opened, Session is created
    • persist() is invoked for entity3, as it's new entity
    • persist() is cascaded to entity1
    • org.springframework.dao.InvalidDataAccessApiUsageException: detached entity passed to persist is thrown
  • Transaction is rolled back, Session is closed

entity1 and entity2 entries are saved in the database.

With @Transactional

As default @Transactional propagation is REQUIRED - there will be only one transaction.

  • entity1, entity2 and entity3 are TRANSIENT now
  • Transaction is opened, Session is created
    • Entity2Repository.save(entity2) is invoked
    • persist() is invoked for entity2, as it's new entity
    • persist() is cascaded to entity1
    • entity1 and entity2 are PERSISTENT now, they are attached to the current Session
    • Entity3Repository.save(entity3) is invoked
    • persist() is invoked for entity3, as it's new entity
    • persist() is cascaded to entity1, managed entity found in the first-level cache
    • entity3 is PERSISTENT now, it is attached to the current Session
  • Transaction is commited, Session is closed

entity1, entity2 and entity3 entries are saved in the database.

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