gistfile1.txt

class Company < ActiveRecord::Base
  has_many :users
end

class User < ActiveRecord::Base
  belongs_to :company
end

class Thing < ActiveRecord::Base
  belongs_to :company
  has_many :users # Can I somehow validate that only users.@each.company_id == self.company_id?
  
  validate :users_belong_to_same_company # Or do I need a custom validation method?
  
  def user_ids=(user_ids)
    # Or do I need to do it here?
  end
end