How to model a mutual friendship in Rails

Question

I know this question has been asked before on Stack Overflow, but the answers aren\'t doing it for me in ways I can explain. My general approach was inspired by this tutoria

Answer 1

This article shows how to set up reciprocal relationships: Bi-directional relationships in Rails

It shows how to use after_create and after_destroy to insert additional relationships that model the reciprocal relationship. In that way, you'd have double the records in your join table, but you'd have the flexibility of using a.friends and b.friends and seeing that both include each other correctly.

Making it work with your model:

class Person < ActiveRecord::Base
  has_many :friendships, :dependent => :destroy
  has_many :friends, :through => :friendships, :source => :person
end

class Friendship < ActiveRecord::Base
  belongs_to :person, :foreign_key => :friend_id
  after_create do |p|
    if !Friendship.find(:first, :conditions => { :friend_id => p.person_id })
      Friendship.create!(:person_id => p.friend_id, :friend_id => p.person_id)
    end
  end
  after_update do |p|
    reciprocal = Friendship.find(:first, :conditions => { :friend_id => p.person_id })
    reciprocal.is_pending = self.is_pending unless reciprocal.nil?
  end
  after_destroy do |p|
    reciprocal = Friendship.find(:first, :conditions => { :friend_id => p.person_id })
    reciprocal.destroy unless reciprocal.nil?
  end
end

I've used this approach successfully on a few projects, and the convenience is fantastic!