exclude association option for ActiveRecord

I've had a number of cases now where I'm working with AR objects that
have associations, but I don't always want to query on the
associations or retrieve them from the database. What about an
option to find() that tells AR to not join on certain associates when
doing a select? I'm thinking of a syntax just like :include which is
used for eager loading. :exclude would tell AR to not do a join on
the models that are excluded.

This has become even more of an issue recently when working with
webservices, because record structure are predefined in the webservice
api and that limits some of the tricks available for working around
this issue.