NHibernate many-to-many using foreign key different to primary key

Question

When creating a one-to-many relationship in NHibernate it is possible to use the "property-ref" attribute to specify that the relationship occurs on a different column to the primary key. This is sometimes useful for legacy databases.

It seems that the same feature should be available for many-to-many relationships, but I can't find it anywhere. Does anyone know if this is possible?

For example, how would I map the following:

table Foo { Id (PK), Name (Unique) }
table Bar { Id (PK) }
table FooBars { FooName, BarId }