プレーンなクラスにActiveRecordのアソシエーションを生やすfake_associationsをつくった

class Tweet < ActiveRecord; end

class User

  attr_accessor :id

  has_many :tweets
end

class User
  include FakeAssociations

  # Need
  attr_accessor :id

  has_many :tweets

  has_many :friendships, foreign_key: 'follower_id'
  has_many :followed_user, through: :friendships, source: :followed

  has_many :reverse_friendships, foreign_key: 'followed_id', class_name: 'Friendship'
  has_many :followers, through: :reverse_friendships, source: :follower

  has_many :favorite_tweets, through: :favorites, source: :tweet
end

class Friendship < ActiveRecord::Base
  #  Table name: friendships
  #
  #  id          :integer  not null, primary key
  #  follower_id :integer
  #  followed_id :integer
end

class Tweet < ActiveRecord::Base
  #  Table name: tweets
  #
  #  id          :integer  not null, primary key
  #  user_id     :integer
end

class Favorite < ActiveRecord::Base
  #  Table name: favorites
  #
  #  id          :integer  not null, primary key
  #  user_id     :integer
  #  tweet_id    :integer
end