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 Post subject: questions on only remove relationship between many-to-many
PostPosted: Fri Aug 18, 2006 2:04 am 

Joined: Thu Aug 17, 2006 11:23 pm
Posts: 1
I am a newbie in hibernate.
Now, I come across one problem.
I want to remove the relation between two entities only. Take for example, I have two entities such as User and Group. They have a many-to-many relationship. Now I want to remove a user from the group. One solution is to load group's user colletion and remove related user object and saveOrUpdate group object finally. But it is not very good. Because it generates many sql statements which will affect the performance greatly. I think it better to write one hql statement instead.
Who can tell me how to write this hql statement?
Thanks a lot.

 Post subject:
PostPosted: Sun Aug 20, 2006 9:57 pm 

Joined: Thu Dec 23, 2004 9:08 pm
Posts: 2008
You cannot do it in HQL as the join table is not mapped as an entity. You can do it using an SQL query easily enough:
<sql-query name="RemoveAssocFromUnloadedEntities">
  delete from User_Group where userid = :userid and groupid = :groupid
Then load this query and call executeUpdate. I can't remember if one of the <return*> tags is mandatory or not, but you should be able to get it working quite quickly.

Note that if you do this and you have either the user or the group in memory at the time, you'll have to refresh them afterwards, essentially wasting the performance benefit of running the query. So I suggest using the normal technique (group.getUsers().remove(user), etc.) when you have one or more of the relevant entities loaded, and only resort to the sql-query when neither entity is in memory.

Code tags are your friend. Know them and use them.

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