In your mapping file you can define a composite key as follows:

<natural-id >
<many-to-one name="b" class="B" ... />
<many-to-one name="c" class="C"... />
</natural-id>


You could also do it this way:


<many-to-one name="b" class="B" unique-key="XXX" />
<many-to-one name="c" class="C" unique-key="XXX"... />


The "unique-key" attribute is poorly documented thus it's use is probably discouraged.

Could somebody tell me what the difference is and which one is the correct one to use.