https://coderwall.com/p/h8zibw

rails_association.rb

class User < ActiveRecord::Base
     #let's just make it simple and only add one column for this model called 'username'
     attr_accessible :username

     #use "follower_id" as the foreign key since by default it will use "user_id" which does 
     #not exist inside the "relationships" table
     has_many :relationships, foreign_key: :follower_id

     #find all user with the ID matching the "followed_user_ids" of the result. If I did not 
     #include "source: :followed_user", it will look for the column "following_id" which is wrong
     has_many :followings, through: :relationships, source: :followed_user

     #here we are just reversing the relationship. Instead of using "follower_id", we now use 
     #"followed_user_id"
     has_many :inverse_relationships, class_name:"Relationship", foreign_key: :followed_user_id

     #this line will look for the "follower_id" based on the result of the "inverse_relationships". 
     #no "source" option is required since we have used the "followers" in the association thus 
     #it looks for a "follower_id" on the "relationships" table which it does
     has_many :followers, through: :inverse_relationships
end

class Relationship < ActiveRecord::Base
     #Here, class_name just means that it's a User model.
     belongs_to :follower, class_name: "User"
     belongs_to :followed_user, class_name: "User"
end