JPA: Is there a way to return the next availble PK

Question

if the field id inside my Entity is like this

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

then when I persist the Entity, a unique random Id is created. Is there a way that I can retrieve the next available PK before persist

Maybe this similar question can be helpful: http://stackoverflow.com/questions/3067986/calling-next-value-of-a-sequence-in-jpa — Javi, Jan 07 '11 at 18:10

score 2 · Answer 1 · answered Jan 07 '11 at 16:47

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There isn't in the spec, although some implementations may provide a means. The problem is the ID is generated by the database in some cases, whether by a sequence or a autoincrement. It's too challenging for JPA to know exactly what to do.

answered Jan 07 '11 at 16:47

Brian Topping

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score 1 · Answer 2 · answered Jan 07 '11 at 17:33

1

Without knowing exactly what you are planning to do with the id when you have it, I found this link that may be helpful. I would write a custom sequence class that just calls the super methods to generate the id but then do what you need to do once you have it.

answered Jan 07 '11 at 17:33

mR_fr0g

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This might be a stupid reason. I was just brainstorm on how I would tackle the problem. I am trying to create user profile. The user allow to upload their picture before click `Create User`. So I want to use the same `userId` for the image name, but I wont know that `userId`, until I `persist` the user. – Thang Pham Jan 08 '11 at 16:50

score 1 · Answer 3 · answered Jan 10 '11 at 14:56

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In EclipseLink you can do this buy,

Number nextId = entityManager.unwrap(Session.class).getNextSequenceNumberValue(MyEntity.class);

answered Jan 10 '11 at 14:56

James

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What is the `Session.class`? Is it an import? – Thang Pham Jan 13 '11 at 05:24
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After look around, is `Session` -> `org.hibernate.Session`. I am using EclipseLink, dont think I have that. – Thang Pham Jan 13 '11 at 09:44

score 0 · Answer 4 · answered Jun 30 '11 at 08:39

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@Harry Pham See the Eclipselink User Guide:

http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Entities/Ids/GeneratedValue

At the bottom it states what James already said. You can use the following code to obtain the next sequence number value:

long id = em.unwrap(Session.class).getNextSequenceNumberValue(Employee.class).longValue();

The Session class is in package org.eclipselink.persistence.Sessions.

But using this is eclipselink proprietary and not JPA.

answered Jun 30 '11 at 08:39

prockel

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I just did a quick test for an entity with generation strategy "IDENTITY". It did not work for me. `getNextSequenceNumberValue()` returned 0, which was not correct. Maybe it works only under certain circumstances, which are not obvious. – prockel Jun 30 '11 at 09:09
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You cannot preallocate an IDENTITY id, it requires an insert. In general I would never recommend IDENTITY, use TABLE or SEQUENCE. – James Jun 30 '11 at 13:46

JPA: Is there a way to return the next availble PK

4 Answers4