Forcing has_many to left join

I'd like for my has_many :conditions clause to refer to columns on
both sides of the association. This would require left joining the
tables, all the time.


I've already whipped up a little patch that allows me to drop the
primary key relationship, so I can include whatever conditions I want.
But it just complains that one of the columns is nonexistent, since I
can't figure out how to require a left-join every time.

Is such a thing possible?