I have 4 tables A, B, C, and D. D contains PK's of A, B, and C, and other extra fields. D does not have its own PK.

I know how to map my POJO's (AClass, BClass, CClass, and BCClass) using the composite-element approach:

<bag name="bcCollection" table="d" >
<key column="a_id" />
<composite-element class="BCClass" >
<many-to-one name="bClass" class="BClass" column="b_id" />
<many-to-one name="cClass" class="CClass" column="c_id" />
<property name="extraAttr" column="extra_field"/>
</bag>

Now, what will my mapping file look like if I use the Map and index-many-to-many approach?

You can't with a tenary relation.

So to confirm, my only choice is to use composite-element?

micheal,
is the doc partially wrong?

Ternary Associations
There are two possible approaches to mapping a ternary association. One approach is to use composite elements (discussed below). Another is to use a Map with an association as its index:

<map name="contracts" lazy="true">
<key column="employer_id"/>
<index-many-to-many column="employee_id" class="Employee"/>
<one-to-many column="contract_id" class="Contract"/>
</map>
<map name="connections" lazy="true">
<key column="node1_id"/>
<index-many-to-many column="node2_id" class="Node"/>
<many-to-many column="connection_id" class="Connection"/>
</map>

Yes you can of course do it that way. What you then can't do anymore though is to store additional information in the association table, unless I am really confused :)

Thanks. A followup question:

In the above ternary relationship, my AClass contains a bag of BCClass. However, I always need to view and manipulate a Collection of CClass given an A and a B. So my object model should really be ABClass containing a bag of CClass. However, I have not figured out how this can be done. My understanding is that a top-level (entity) class needs to have an id/PK, but the BCClass does not have an id.

Any help is greatly appreciated!