Rails (ActiveRecord) で scope と joins する方法

答え

ActiveRecord::SpawnMethods#merge を使うらしい。

動作確認

前提

db/migrate/20140410025937_create_authors.rb

class CreateAuthors < ActiveRecord::Migration
  def change
    create_table :authors do |t|
      t.string :name

      t.timestamps
    end
  end
end

db/migrate/20140410030000_create_books.rb

class CreateBooks < ActiveRecord::Migration
  def change
    create_table :books do |t|
      t.integer :author_id
      t.string :title
      t.datetime :published_at

      t.timestamps
    end
  end
end

app/models/author.rb

class Author < ActiveRecord::Base
  has_many :books
end

app/models/book.rb

class Book < ActiveRecord::Base
  belongs_to :author
  scope :published, -> { where(arel_table[:published_at].not_eq(nil)) }
end

実行

$ bundle exec rails c
Loading development environment (Rails 4.0.3)
irb(main):001:0> Author.joins(:books).merge(Book.published)
  Author Load (0.1ms)  SELECT "authors".* FROM "authors" INNER JOIN "books" ON "books"."author_id" = "authors"."id" WHERE ("books"."published_at" IS NOT NULL)
=> #<ActiveRecord::Relation []>

参考文献

• Rails 3 merging scopes with joins - Stack Overflow