I'd like to know how to map a many-to-many association with one property.

For example we have the two tables A and B, and there is a many-to-many association between these two tables that gives us a new table called AB.

In AB there is the primary key from A and from B. And we also find one property in this association.

In the Hibernate reference doc I saw something like this :
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
</composite-element>
</set>

But I'm not sure that is what I need. I tried this but I have some problems maybe because I made some mistakes when I implemented this solution.

Then if this the solution or if you have another solution, could you give me a simple example with the three tables I quoted (A, B and AB) ?


Thanks for your help.

Nicolas

Say A: Teams, B: Users
Many users belong to a team and a user is part of many teams

in mapping for teams:
<set name="users" table="users_teams">
<key column="teamName"/>
<many-to-many column="loginId" class="User"/>
</set>

in mapping for users:
<set name="teams" table="users_teams">
<key column="loginId"/>
<many-to-many column="teamName" class="Team"/>
</set>

So you will get a extra table users_teams with (loginId, teamName) fields

Hope this helps,

--

oops... sorry.. I did not pay much attention to the problem of a common property.. could you be more specific with your problem, a simple association diagram will help me understand better.

--
P.S: Visit this url for a fabulous example of mapping relationships:
http://www.xylax.net/hibernate/index.html

Thanks for helping me :


------ ------- -------
A AB B
------ ------- -------
id_A id_A id_B
id_B
a_property



Relations : To one A there are several AB
To one B there are several AB
To one AB there is one A and one B

AB is the result of the many-to-many association between A and B
(To one A there are serveral B and to one B there are several A).

And a_property is a simple property like a name (String) or something like this (not a key).


But I success to implement the following mapping (see reference doc of Hibernate) :
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
</composite-element>
</set>

and it seems to be the appropriate solution. But it's not sure I have to test it now with some records so I can't say if it is the right solution or not.

After testing it, if I see that's the right solution I will add a reply to my post in order to inform you.


Thanks,

Nicolas

Sorry, maybe the display will be better like this :


------
A
------
id_A

-------
AB
--------
id_A
id_B
a_property


-------
B
-------
id_B



Relations : To one A there are several AB
To one B there are several AB
To one AB there is one A and one B

AB is the result of the many-to-many association between A and B
(To one A there are serveral B and to one B there are several A).

And a_property is a simple property like a name (String) or something like this (not a key).


But I success to implement the following mapping (see reference doc of Hibernate) :
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
</composite-element>
</set>

and it seems to be the appropriate solution. But it's not sure I have to test it now with some records so I can't say if it is the right solution or not.

After testing it, if I see that's the right solution I will add a reply to my post in order to inform you.


Thanks,

Nicolas

the following mapping (see reference doc of Hibernate) :
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
</composite-element>
</set>
is the solution.

But you'll have to map the table AB twice (two classes with distinct names to map the table AB) otherwise you can't access AB by A and B.

Try this and you'll understand what I mean.

Map AB as an entity, not a composite element.
A 1-n AB n-1 B

The best solution was well in the reference documentation, I success to implement it :

Reference documentation
"A mapping like this allows you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association from Order to Item where purchaseDate, price and quantity are properties of the association:

<class name="eg.Order" .... >
....
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
</composite-element>
</set>
</class>
"


Another solution consists to do like this :

in A declares a one-to-many with table AB
in AB declares a many-to-one with table A and a many-to-one with table B
in B declares a one-to-many with AB

But the best solution is to do as I first explained