gistfile1.rb

class Company
  include Mongoid::Document
  field :user_ids, :type => Array, :default => []
  field :admin_ids, :type => Array, :default => []

  def users
    User.where(:_id.in => self.user_ids)
  end
  
  def admins
    User.where(:_id.in => self.admin_ids)
  end
end

class User
  include Mongoid::Document

  def companies
    Company.where(:user_ids => self.id)
  end
end