I have two tables A and B as follows

A : a_id(PK) , a_collectionid
B : b_id(PK) , b_data

One more table AB is as follows:

AB : a_collectionid , b_id ,ab_order(index of the collection) ,ab_id(PK).

This is like a many-to-many relationship between A and B.
more like A has a collection of Bs.
From what i understand from the forums, this cannot be expressed in the mapping document using property-ref.
What are the possible workarounds for this case?

Thanks,
Lakshmi.