JPA: foreign key annotation

Question

I've got two database entities: Forum and Topic.

Topic has protected long forumId data member, which indicates of course the Topic's forum.

My question is what annotation to use for this data member?

why do you say so? I think it's necessary because I have to know which forum the topic belongs to — socksocket, Aug 10 '12 at 16:53
@socksocket I think it's because you can annotate an attribute `Forum`, specifying the id with a `@JoinColumn` annotation, see my answer. ;) — Elias Dorneles, Aug 10 '12 at 16:54

score 51 · Accepted Answer · answered Aug 10 '12 at 16:51

51

As Forum has many topics, and a topic belongs to one and only Forum, you probably want to go with a Forum type attribute annotated with @ManyToOne:

@ManyToOne
@JoinColumn(name = "forumId")
private Forum forum;

See more:

ManyToOne and JPA mapping

answered Aug 10 '12 at 16:51

Elias Dorneles

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but the "problem" in this case is that when you serialize your Topic to XML or JSON (to pass it to a client, for example), the whole Forum object will also be serialized by default... same thing if you upload a topic to your server, you need to pass the whole Topic+Forum objects, you cannot just use the forum id. In some cases, it seems easier to just manually insert the foreign key of the related entity. – splinter123 Sep 09 '14 at 22:45
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How to solve only the forumId in your example/case?? – stallion Aug 12 '18 at 09:08
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@stallion if you configure lazy initialization, you will get only the id (no join required) if you only do `getForum().getId()`. See: https://stackoverflow.com/questions/32220951/just-getting-id-column-value-not-using-join-in-hibernate-object-one-to-many-rela/32223785#32223785 – Elias Dorneles Aug 13 '18 at 11:00

score 5 · Answer 2 · edited Jul 29 '18 at 11:58

As others have answered -
You should use the ManyToOne , and JoinColumn annotations.
Bare in mind , that since JPA is about ORM - Object relational mapping,
You should reference another object as you would have done "naturally" in Java - i.e via an object and not via its identifier (which is forumId) in your case),
This was one of the design consideration between the relations at JPA and Hibernate (previously to JPA).

score 1 · Answer 3 · edited Jul 29 '18 at 11:58

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@ManyToOne As the annotation implies - you have many topics per forum

edited Jul 29 '18 at 11:58

Michael Lihs

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answered Aug 10 '12 at 16:52

munyengm

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JPA: foreign key annotation

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