ActiveRecordでid以外によるアソシエーションを設定する

foreign_key: と primary_key: オプションを利用する。

# == Schema Information
#
# Table name: twitter_accounts
#
#  id                                 :bigint(8)        not null, primary key
#  screen_name                        :string(255)      not null
#
# Indexes
#
#  index_twitter_accounts_on_screen_name  (screen_name) UNIQUE
#
class TwitterAccount < ActiveRecord
  validates :screen_name, presence: true, uniqueness: true
  has_many :users, foreign_key: :twitter_screen_name, primary_key: :screen_name
end

# == Schema Information
#
# Table name: users
#
#  id                                 :bigint(8)        not null, primary key
#  twitter_screen_name                :string(255)
class User < ActiveRecord
end

twitter_account = TwitterAccount.first
twitter_account.users.to_sql #=> "SELECT `users`.`id`, `users`.`twitter_screen_name` FROM `users` WHERE `users`.`twitter_screen_name` = 'takeyuweb'"