In my particular case, I may not even need to worry about the has_many
relationship from A to B. If all I need to take advantage of is the
fact that B belongs_to A, _and_ I'm sure that my keys are all
consistent, then simply doing
belongs_to :a, :foreign_key => 'appropriate_foreign_key' will be fine
since "belongs_to" only generates a query on the parent table and isn't
trying to talk to both model tables at once.
And even if I do need the has_many relationship, it is easiest to simply
define the appropriate "bs" (that's as in the plural of "b") method on A
which can just go directly to the B table and filter it based on the
None of this cross-DB stuff is really necessary.