class User < ActiveRecord::Base

  module Friends

    def common(user)
      # Define code here

    end
  end


  has_and_belongs_to_many :friends,
                          :class_name => 'User',
                          :association_foreign_key => 'friend_id',
                          :join_table => 'friends_users',
                          :extend => User::Friends

  def self.find_common(u1, u2)
    find_by_sql([<<-EOM, u1.id, u2.id])
      SELECT u.*
      FROM users u
      LEFT JOIN friends_users fu1 ON fu1.user_id = ? AND fu1.friend_id = u.id
      LEFT JOIN friends_users fu2 ON fu2.user_id = ? AND fu2.friend_id = u.id
      WHERE fu1.user_id IS NOT NULL AND fu2.user_id IS NOT NULL
      ORDER BY u.first_name
    EOM
  end
end