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Re: hibernate: mapping he

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Matteo wrote:
> I'm stuck with my implementation of the "friendship" relationship, as I 
> decided to define it with a directed-graph, thus user.A and user.B are 
> "friends" if and only if
> ( FRIENDS.userID = user.A ; FRIENDS.friendID = user.B ) AND  ( 
> FRIENDS.userID = user.B ; FRIENDS.friendID = user.A )
> Hence, the composite-id (userID, friendID) of FRIENDS table.

The "Hence" clause doesn't follow - that would be the primary key even if you 
define a "friend" as any row in the FRIENDS table, even if the converse row is 
absent.  There is no connection between the "AND" of the rule you stated and 
that primary key.  In fact, nothing in the data definition you showed enforces 
that "A" must appear in two rows of the table, once as the "userID" and once 
as the "friendID".  I wouldn't bother enforcing that, either.

> As for MESSAGES table, both FROM and TO fields referentiate USERS.userID 
> (and I'm stuck with that too )

There's nothing to be "stuck with" there - it's a perfectly reasonable data 
design.

Is this a MySQL database?  You didn't say.

Couldn't you just map the tables in the very straightforward way?  The 
relation between USERS and USERS is many-to-many.  Seems like you could fake 
it as a one-to-many, though.

I don't know Hibernate well, but it seems like using <entity-name> might help.
<http://www.hibernate.org/hib_docs/v3/reference/en/html/mapping.html#mapping-entityname> 


The use of a join table looks like you need to read
<http://www.hibernate.org/hib_docs/v3/reference/en/html/associations.html#assoc-bidirectional-join>

I found pretty much your exact problem mentioned in 2005:
<http://objectmix.com/jdbc-java/41501-hibernate-persons-friends-newbie-mapping-question.html>

-- 
Lew

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hibernate: mapping help "Matteo" <matteo@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000
  Re: hibernate: mapping he "Lew" <lew@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000
    Re: hibernate: mapping he "Matteo" <matteo@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000
      Re: hibernate: mapping he "Lew" <lew@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000
        Re: hibernate: mapping he "Matteo" <matteo@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000
          Re: hibernate: mapping he "Lew" <lew@THRWHITE.remove-dii-this> - 2011-04-27 15:21 +0000

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