Some third party modules expects that association returns same proxy
object each time (e.g. for stubbing collection methods:
https://github.com/rspec/rspec-rails/issues/1817).
So I decided that cache the proxy object and reset scope in the proxy
object each time.

Related context: https://github.com/rails/rails/commit/c86a32d7451c5d901620ac58630460915292f88b#commitcomment-2784312