Bidirectional Relation only working one way

odennis · **Joined:** Thu Dec 21, 2006 11:22 am **Posts:** 2

Hi there, I'm fairly new to hibernate but have so far managed quite well with mapping objects and pulling stuff from my mysql database.

I'm having trouble with a particular relationship in my model, I have a class called group and another called category, what I'm trying to achieve is one-to-many relationship so that one group can have many categories, the relation also needs to be bidirectional so I can pull the group from any given category.

Here is the relation in my mapping files, this one from category:

Code:

<many-to-one name="group" cascade="all" lazy="false" class="tracker.persistent.Group" not-null="true" foreign-key="fk_cat_group">
            <column name="group_fk" />
</many-to-one>

And this one from group:

Code:

<set name="categories" table="t_category" lazy="false" cascade="all" inverse="true" >
            <key column="group_fk"/>
            <one-to-many class="tracker.persistent.Category" />
</set>

The relation works from category to group however when I execute the relation the other way (i.e. group.getCategories) I end up with a null set which I know from executing standard sql against the database is not the case.

I'm using mysql 5 with hibernate 3

Any help would be gratefully received,
Many thanks,
Oli

batmat · **Posted:** Thu Dec 21, 2006 1:25 pm

Are you sure the cascade="all" attribute on the <many-to-one> is wanted? It's saying to Hibernate that if you delete a category, it should cascade deletion to its parent ?!

Moreover, putting cascading on both side is quite unusual. IMO, you should remove this cascade="all", so as your mapping is something like :

Code:

<many-to-one name="group" lazy="false" not-null="true" foreign-key="fk_cat_group" column="group_fk" />

Javier Martinez · **Posted:** Thu Dec 21, 2006 6:11 pm

You had better change the "set tag" for something like this:

Code:

<set name=" categories" inverse="true">
   <key column="group_fk"/>
   <one-to-many class=" tracker.persistent.Category "/>
</set>

Best regards.

odennis · **Joined:** Thu Dec 21, 2006 11:22 am **Posts:** 2

Hi there,

thanks for your help so far however I'm still having the same problem, whenever I do goup.getCategories I have a null object returned, should this really be the case or should I get back an empty set?

In any case I should be getting a populated set as doing a direct sql statement against the db returns 25 child categories for my given group.

Any thoughts and advice gratefully recieved,
Thanks again,
Oli

batmat · **Posted:** Wed Dec 27, 2006 4:59 pm

Did you try the mapping Javier Martinez and I provided? You say that using SQL, you're sure that the given group has categories: did you do it using the query Hibernate generates? Is this generated query correct or not?

Using log4j: you should activate the equivalent of show_sql="true" in hibernate.cfg.xml => org.hibernate.SQL

=> add this to your log4j.properties file

Code:

log4j.logger.org.hibernate.SQL=debug
log4j.logger.org.hibernate.bind=debug

This will let you see the exact query Hibernate sends and the value it uses for parameters.

If the query is correct, then I can't see why Hibernate wouldn't give you your 25 instances.

About null or empty set, I don't remember. I'd say null, but I also know that Hibernate uses something like ArrayList.EMPTY_SET sometimes. I'll try and check and let you know.