If I have a table A, with column A1 and A2 and a table B with column B1. The association between A and B is B1 = A1 and A2 = 'some constant value'

How can I map this with Hibernate?

It appears that I can have associations on multiple columns by nesting a properties element, but I haven't been able to find a way of specifying that some column must have a constant value.

Hope the question make sense.

That's a weird one. If the constant value was on the contained table, you'd use <set where="column=value" ...>, but if the constant value is on the containing table.. I think that's not possible. Wouldn't it be a discriminator in that case?