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From Srinivasan Krishnamoorthy <>
Subject Re: Conceptual question on ManyToMany Relation
Date Thu, 08 Oct 2009 10:18:06 GMT

Hi Wes,
 Thanks for suggesting this solution.. I agree thats the best way to 
model this scenario .. so the one liner I would take is "a big no for any
Many-To-Many mappings
where the entities can theoretically grow to large numbers".. 

Srinivasan Krishnamoorthy.


Wes Wannemacher wrote:
> I have an app where I am modelling users/groups, but instead of
> leaving it up to the JPA implementation, I created my own junction
> table via an intermediary entity. That way, I just have a one-to-many
> mapping from users to the UserGroup entity and a one-to-many from
> groups to UserGroup. To make it easier, I added @Transient methods to
> get a users' groups and the groups' users. This way I can closely
> control the behavior.
> You definitely want to use Lazy fetch groups where ever possible in
> this sort of setup because loading one user could conceivably load all
> of the data from all of the tables.
> On Tue, Oct 6, 2009 at 11:57 AM, Srinivasan Krishnamoorthy
> <> wrote:
>> Hi,
>>  I have difficulty in understanding the JPA implementation of ManyToMany
>> relation, conceptually.
>> Let me try to put forward the question with an example.
>> I have a many to many relation between users and groups.
>> My use case is, I want to add a user to a group. Ideally, with SQL all
>> that
>> I would be doing is create a new link in a link table. But with JPA, I
>> see
>> the following steps to be done.
>> 1. Get a group object, and add the user to the 'User Set'. (If I have
>> 1000
>> users I need to fetch them all).
>> 2. Add the group to 'Group Set' of User Object. (Here too, I need to
>> fetch
>> all the groups, say 100).
>> 3. JPA will do a 'merge' to find just the extra link and save it.
>> My question is, why should I do so much to just add a link? Is there an
>> easier way to do this.. or is my understanding wrong.. Can you please
>> share
>> your thoughts on this?
>> Also, I am afraid, what will happen if both sides of ManyToMany has huge
>> data (theoretically).
>> (PS: With OneToMany I can work around this issue by modeling the system
>> to
>> always work from Many side, but dont have an idea for ManyToMany)
>> Best Regards,
>> Srinivasan Krishnamoorthy.
>> --
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> -- 
> Wes Wannemacher
> Head Engineer, WanTii, Inc.
> Need Training? Struts, Spring, Maven, Tomcat...
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