Using JPA to partially initialize entities

Posted on 2011-02-17
Last Modified: 2013-11-24

I have created an application with a relatively complex model. Everything was working fine and the beta-tests were running smoothly. The problem I am having is that I am using BlazeDS to send results to a Flex client. This causes all relations to be loaded (even if they are marked for lazy loading) and sent to the client. This is rather undesirable as a simple listEntityA doesn't only return a list of EntityA but also all of their references and the references of their references ... a lot of stuff.

I know in JPA I can do these really cool "SELECT NEW packagename.Classname(params...) FROM ..." to create unmanaged instances that are partially initialized (projections). The problem is, that I really would like the main entity to be fully initialized, but I would love some related objects to be only initialized with let's say "id" and "name" (all other properties left empty) ... this would be a projection of the relation.

How could I do this?
Question by:ChristoferDutz
LVL 47

Expert Comment

ID: 34915055
Would not claim that I understand much of what you are writing about, but this is the link with quite similar words,
who knows, maybe you haven't seen it:
LVL 20

Author Comment

ID: 34915254
I just added the Flex, BlazeDS, LazyCollections words in order to set the focus. My system works great without any errors. I am currently optimizing. Let me explain my problem in a little more detail

I have the following entities with the properties:

- id
- name
- oneToMany to EntityB
- oneToMany to EntityC

- id
- name
- oneToOne to EntityD

- id
- name
- (big blob of data)

- id
- name
- (big blob of data)

Open in new window

Now immagine I want to load a list of EntityA objects. When displaying one of the EntityA Objects in a list, I want to output the name of all EntityBs that are associated with that entity and output a link behind the name to switch to that entities view (I only need the id and name and not all the other properties).

So I am looking for a way to do the projection select on the related objects and not the main entity.

Expert Comment

ID: 34925099
What I understood is u want to load only the parent record but not the child record. If that is the case then you can use lazy loading

@OneToMany(mappedBy = "type", fetch = FetchType.LAZY)
    private List<Type> type= new ArrayList<Type>();
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LVL 20

Author Comment

ID: 34925118
No ... I want to load the parent and I want to partially load the children.
I can load an entity partially by using projection in my select-statement "SELECT NEW de.cware.cweb.model.User(, FROM User usr"
What I would like to do, ist to load the parrent fully and the related entities partially in a way that in the related objects only the id and name properties are initialized.


Accepted Solution

aciuica earned 500 total points
ID: 34928956
Maybe it depends of which JPA implementation you use (if supports fields lazy loading), but annotating some fields from related entities with lazy loading can be a soution.
LVL 20

Author Comment

ID: 34954489
I'm currently stuck with an ANTLR problem ... I'l check this lazy loaded property thing you mentioned ... sounds very prommissing :-)
LVL 20

Author Closing Comment

ID: 34996366
Thanks for the tip with the lazy-loaded properties.

I actually solved my problem by going from JPA to Jdbc directly as here I could controll exactly what I wanted to do.

Thanks anyway.

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