hibernate - how to save parent with detached child

Question

I am sending an object from UI. This object is going to be created with a reference to an existing child.

This is simple illustration for this relation.

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Answer 1

Just use the proxy for the child:

parentEntity.setChild(entityManager.getReference(ChildEntity.class, childId));

The point here is to use EntityManager.getReference:

Get an instance, whose state may be lazily fetched.

Hibernate will create the proxy containing only the id without going to the database.