sferik/rails_admin — pull request #2073 by tsukasaoishi (2014-09-27 16:01:48+00:00)
Title: Column#primary was removed at Rails4.2

https://github.com/rails/rails/commit/05dd3df35db8b2d39ed177f4ccfe112bdfe7726d

Column#primary is undefined method at Rails4.2.
Use ActiveRecord::Base.primary_key Instead of Column#primary