[jpa] JPA EntityManager: Why use persist() over merge()?

Either way will add an entity to a PersistenceContext, the difference is in what you do with the entity afterwards.

Persist takes an entity instance, adds it to the context and makes that instance managed (ie future updates to the entity will be tracked).

Merge returns the managed instance that the state was merged to. It does return something what exists in PersistenceContext or creates a new instance of your entity. In any case, it will copy the state from the supplied entity, and return managed copy. The instance you pass in will not be managed (any changes you make will not be part of the transaction - unless you call merge again). Though you can use the returned instance (managed one).

Maybe a code example will help.

MyEntity e = new MyEntity();

// scenario 1
// tran starts
em.persist(e); 
e.setSomeField(someValue); 
// tran ends, and the row for someField is updated in the database

// scenario 2
// tran starts
e = new MyEntity();
em.merge(e);
e.setSomeField(anotherValue); 
// tran ends but the row for someField is not updated in the database
// (you made the changes *after* merging)
      
// scenario 3
// tran starts
e = new MyEntity();
MyEntity e2 = em.merge(e);
e2.setSomeField(anotherValue); 
// tran ends and the row for someField is updated
// (the changes were made to e2, not e)

Scenario 1 and 3 are roughly equivalent, but there are some situations where you'd want to use Scenario 2.

Examples related to jpa

No converter found capable of converting from type to type How does spring.jpa.hibernate.ddl-auto property exactly work in Spring? Deserialize Java 8 LocalDateTime with JacksonMapper Error creating bean with name 'entityManagerFactory' defined in class path resource : Invocation of init method failed How to beautifully update a JPA entity in Spring Data? JPA Hibernate Persistence exception [PersistenceUnit: default] Unable to build Hibernate SessionFactory How to return a custom object from a Spring Data JPA GROUP BY query How to find distinct rows with field in list using JPA and Spring? What is this spring.jpa.open-in-view=true property in Spring Boot? Spring Data JPA and Exists query

Examples related to merge

Pandas Merging 101 Python: pandas merge multiple dataframes Git merge with force overwrite Merge two dataframes by index Visual Studio Code how to resolve merge conflicts with git? merge one local branch into another local branch Merging dataframes on index with pandas Git merge is not possible because I have unmerged files Git merge develop into feature branch outputs "Already up-to-date" while it's not How merge two objects array in angularjs?

Examples related to entitymanager

What does EntityManager.flush do and why do I need to use it? JPA With Hibernate Error: [PersistenceUnit: JPA] Unable to build EntityManagerFactory The EntityManager is closed What is the best way to update the entity in JPA Create JPA EntityManager without persistence.xml configuration file JPA EntityManager: Why use persist() over merge()?

Examples related to persist

PersistentObjectException: detached entity passed to persist thrown by JPA and Hibernate JPA EntityManager: Why use persist() over merge()?

Examples related to java-persistence-api

Difference between FetchType LAZY and EAGER in Java Persistence API? JPA EntityManager: Why use persist() over merge()?