rails/rails — pull request #18174 by bogdan (2014-12-23 19:32:16+00:00)
Title: Remove unneeded special case to calculate size for has_many :through

All cases are properly handled in CollectionAssociation for all subclasses of this association.

I was trying to investigate the issue fixed in https://github.com/rails/rails/commit/f045663dfc879b9516904255fb555f048c394b7b in 4.1 stable and realised that this method is not required right now.