Addition to has_many - a ':source_scope' option

I think I'd rather see scope than source_scope.

But that's really bikeshedding.

+1 for this. It would be nice if you could also throw in support for :unscoped (to counter the effect of a default_scope on the association), but I suppose even if this doesn't support that out of the box I could always make an "unfiltered" scope on the association.