lazy loading

someone12345 · **Joined:** Sun Apr 16, 2006 1:02 pm **Posts:** 12

Hi!

I don't quite understand Hibernate behaviour of loading entities. Two simple Tables: html and text (extracted text from html):

Code:

 public class Html 
 { 
    @OneToOne( fetch = FetchType.LAZY, cascade = CascadeType.ALL ) 
    @PrimaryKeyJoinColumn 
    private Text text;
   

Now I want to select all HTML records that haven't been parsed yet:

Code:

FROM Html h WHERE h.urlId != ALL (SELECT urlId FROM Text)

This yields to a single SQL statement that does what it's supposed to do but also each single Text row to be loaded. If I remove the Text relation from Html everything's fine.

krawling · **Posted:** Thu Feb 08, 2007 11:23 am

Hello,

I was just working with a similar one-to-one relationship which shares the same PK. I experienced the same behavior as you mentioned regardless of the FetchType setting.

I don't know the reason behind this behavior but I was experimenting with the hibernate extension

Code:

@org.hibernate.annotations.Fetch(
    org.hibernate.annotations.FetchMode.JOIN
)

The data from the secondary row will still load but hibernate will execute a single joined select.

Regards,
Kurt

someone12345 · **Joined:** Sun Apr 16, 2006 1:02 pm **Posts:** 12

Hmm, nope, unfortunately not

Code:

public class GzHtml
{
   @Id
   @Column( name = "url_id" )
   private String urlId;

   @Column( name = "html" )
   @NotNull
   private byte[] html;

   @OneToOne( fetch = FetchType.LAZY, cascade = CascadeType.ALL )
   @PrimaryKeyJoinColumn
   @org.hibernate.annotations.Fetch( org.hibernate.annotations.FetchMode.JOIN )   
   private GzText gzText;

Code:

Hibernate: 
    /* load f4t.model.GzText */ select
        gztext0_.url_id as url1_2_0_,
        gztext0_.title as title2_0_,
        gztext0_.summary as summary2_0_,
        gztext0_.text as text2_0_ 
    from
        public.gztext gztext0_ 
    where
        gztext0_.url_id=?
Hibernate: 
    /* load f4t.model.GzText */ select
        gztext0_.url_id as url1_2_0_,
        gztext0_.title as title2_0_,
        gztext0_.summary as summary2_0_,
        gztext0_.text as text2_0_ 
    from
        public.gztext gztext0_ 
    where
...

christian · **Posted:** Thu Feb 08, 2007 11:52 am

If you type "lazy one-to-one" into the search field on the Hibernate website, you get this: http://www.hibernate.org/162.html

krawling · **Posted:** Thu Feb 08, 2007 12:05 pm

Christian,

That link provides a good explanation, thanks for the response.

Regards,
Kurt

someone12345 · **Joined:** Sun Apr 16, 2006 1:02 pm **Posts:** 12

Yes (I even did read the page but apparently not thoroughly). Unfortunately it doesn't solve the problem :\

So, this means another use-case specific domain model might be required, e.g. if the use-case requires to quickly read a vast amount of contrained data without all possible (but use-case useless) relations resolved. E.g. when extracting content from an corporate CMS in order to build a search index. Or some @hack annotations :)

BTW is there some page helping to map .hbm.xml @annotations?

christian · **Posted:** Thu Feb 08, 2007 12:46 pm

Or it could be that...

- Your FetchMode.JOIN didn't work because you also enabled FetchType.LAZY. Use FetchType.EAGER + FetchMode.JOIN if you want eager join fetching, or FetchType.EAGER + FetchMode.SELECT if you want eager fetching with a secondary select. This is also explained in the reference documentation(s) and there is even another extra page on the Wiki about it for people who don't like documentation. It's called "A primer on fetching strategies".

- If you still want FetchType.LAZY, and proxies don't work for you (and shared PK one-to-zero-or-one is such a rare exception), use @LazyToOne and run the bytecode enhancer to inject interception code instead. You can find this in the reference documentation as well.

someone12345 · **Joined:** Sun Apr 16, 2006 1:02 pm **Posts:** 12

christian wrote:

- Your FetchMode.JOIN didn't work because you also enabled FetchType.LAZY. Use FetchType.EAGER + FetchMode.JOIN if you want eager join fetching, or FetchType.EAGER + FetchMode.SELECT if you want eager fetching with a secondary select.

Maybe I got this wrong but this still loads the Text entities with a bunch of single selects.

christian wrote:

- If you still want FetchType.LAZY, and proxies don't work for you (and shared PK one-to-zero-or-one is such a rare exception), use @LazyToOne and run the bytecode enhancer to inject interception code instead. You can find this in the reference documentation as well.

I remember having read @LazyToOne but the bytecode enhancer detered me. But certainly a better solution than @hack annotations :) I'm going to try this tomorrow.

someone12345 · **Joined:** Sun Apr 16, 2006 1:02 pm **Posts:** 12

Hm. I'd a shot an @LazyToOne but it still loads all these Text entities. Yes, I ran the cglib enhancer and I even decompiled the class:

Code:

    public GzText getGzText()
    {
        return $cglib_read_gzText();
    }

Code:

   // http://www.hibernate.org/162.html
   @OneToOne( fetch = FetchType.LAZY, cascade = CascadeType.ALL, optional = true )
   @LazyToOne( value = LazyToOneOption.PROXY )
   @PrimaryKeyJoinColumn
   private GzText gzText;