Need Help Mapping Latest Encapsulated Entity

oakridge · **Joined:** Fri Jul 09, 2010 1:24 pm **Posts:** 11

Hi all,

Here's is a situation I am encountering. Let's say I have a Blog entity with a field called latestPosting, which is of type Post. This latest posting field should be a unique entry where the latestPosting is the newest record in the database and I cannot use the date field on the Post. I would like to have Hibernate retrieve this field as part of a JPA or Hibernate annotation. I also need to have this work with lazy loading. In other words, I can't depend on a Hibernate @Filter that is enabled at the session level during retrieval. I found this solution online, but it requires a OneToMany mapping. I'd need a OneToOne. Also, take note that I also have mapped a collection of Posting elements in my Post entity.

http://venky-techspace.blogspot.com/201 ... ilter.html

So my questions are:

Is this achievable via an annotations only approach?
If not, should I use a read-write view?

I am using Hibernate 3.5.6, Oracle 10g, mixed JPA and Hibernate Annotations, and Java 6.

Here is a brief sample of what the Blog model look likes:

Code:

@Entity
@Table(name="BLOG")
public class Blog{

@Id
@Column(name="id")
private Integer id;

//??? what mapping can we use here
private Post latestPosting;

@OneToMany(mappedBy = "blog")
private List<Post> postings;

......

}

The Post model:

Code:

@Entity
@Table(name="POST")
public class Post{

@Id
@Column(name="id")
private Integer id;

@ManyToOne
@JoinColumn(name="blog_id")
private Blog blog;

......

}

oakridge · **Joined:** Fri Jul 09, 2010 1:24 pm **Posts:** 11

bump...anyone have some pointers?

saltnlight5 · **Posted:** Wed Sep 22, 2010 2:56 pm

So go use the @OneToOne on Blog entity. What's the problem?

Code:

//??? what mapping can we use here
@OneToOne
@JoinColumn(name="post_id")
private Post latestPosting;

Now when you do that, you just have to update the post in blob entity whenever there is a latest post. A better way probably just use the fact you have a

Code:

List<Post> postings;

, go ahead and order it in whatever way you think it consider latest, then your latest Post IS simply just

Code:

postings.get(0);

oakridge · **Joined:** Fri Jul 09, 2010 1:24 pm **Posts:** 11

Quote:

So go use the @OneToOne on Blog entity. What's the problem? Now when you do that, you just have to update the post in blob (blog?) entity whenever there is a latest post.

The problem is that this is not an option. I provided this as a sample model. The actual model I am working with doesn't deal with Blogs and Posts. Sorry for the obfuscation, but I was attempting to keep it simple. Regardless, the real issue is that I need Hibernate to be aware of how this property is derived. If Hibernate is aware of it via a JPA/Hibernat annotation, then I can use that property in Criteria queries to order a collection of "Blogs" by their latest Posts. Also, I would be able to query all Blogs with a latest posting date of XYZ. Finally, I wouldn't have to load the entire collection to find the latest posting.

I suppose I could use JPQL to do all of this, but it would be an exception to my framework that I would like to avoid. Currently, the framework doesn't have any special use cases coded within it. It only deals with requests to sort and filter based on property names alone, which are provided to a Criteria for Hibernate to digest. With this in mind, I can't do:

Code:

postings.get(0);

I appreciate the input and welcome any other ideas.

****UPDATE****
It turns out that max(id) will not guarantee me the lastest posting. I actually have to retrieve it by a date field on the post (e.g. max(postDate) )

oakridge · **Joined:** Fri Jul 09, 2010 1:24 pm **Posts:** 11

Any thoughts on this? It would seem Hibernate cannot handle my use case, but I am hoping a Hibernate guru can confirm or deny this.

Thanks!