Planned lessonMID-021-L04

Entity states revisited: merge vs persist

On This Lesson 4 topics

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

handle detached entities correctly.

States: transient (new), managed (in the persistence context), detached (loaded earlier, session closed), removed.
persist makes a new entity managed; merge copies the state of a detached entity onto a managed copy and returns that copy.
Use the returned object from merge; the argument stays detached.
Spring Data save calls persist for new entities and merge otherwise; merge of an unknown id may do an extra SELECT.
Prefer loading the entity inside the transaction and changing it (dirty checking) over merging DTO-built entities.

Draft outline details

java

Product managed = em.merge(detached);   // use 'managed', not 'detached'

What does merge return?

Show answer

The managed copy; the passed object stays detached.

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