See the minitest to demonstrate the problem:
https://gist.github.com/nagyt234/b4a9bb210f7cf83262ab
The generated SQL contains

AND "documents_revisions"."code" = 'BBB'

instead of

AND "documents"."code" = 'BBB'

The problem appears only if the default scope on model Revision applied and the first condition refer to an other association.
Note, that this error cames up on Rails 4.1.2 (used in my project), on the newest Rails 4.2 and on Rails 5 beta either.