rails/rails — pull request #17859 by scambra (2014-11-30 22:37:34+00:00)
Title: Includes HABTM returns correct size now

It's caused by the join dependency, only instantiates one HABTM object because the join table hasn't a primary key.
Updated commit from @bigxiang commit dbaa837

Fixes #16032.

Examples:

before:

Project.first.salaried_developers.size # => 3
Project.includes(:salaried_developers).first.salaried_developers.size # => 1

after:

Project.first.salaried_developers.size # => 3
Project.includes(:salaried_developers).first.salaried_developers.size # => 3