class User < ActiveRecord::Base
      enumerize :status, :in => { active: 1, blocked: 2 }, scope: true
   end
...
    it 'has correct value in _was attribute in child class' do
      class AdminUser < User
        enumerize :status, :in => { active: 1, blocked: 2, inactive: 3 }, scope: true
      end

      admin = AdminUser.create(status: 'active')
      admin.update status: 'blocked'
      expect(admin.status_was).must_equal 'active'
    end