【Rails】scopeメソッドとクラスメソッドでnilを返す時の挙動が違う

クラスメソッドの場合

class Book
  def self.published
    where(published: true)
  end
end

Book.published.order(id: :desc)
#=> Attendance Load (3.8ms)  SELECT `attendances`.* FROM `attendances` WHERE `attendances`.`date` BETWEEN '2022-01-01' AND '2022-01-31' ORDER BY `attendances`.`id` DESC

nilの場合はnilが返るのでメソッドチェーンが使えない。

class Book
  def self.published
    nil
  end
end

Book.published
#=> nil

Book.published.order(id: :desc)
#=> NoMethodError: undefined method `order' for nil:NilClass

scopeの場合

class Book
  scope :published, -> { where(published: true) }
end

Book.published.order(id: :desc)
#=> Attendance Load (3.8ms)  SELECT `attendances`.* FROM `attendances` WHERE `attendances`.`date` BETWEEN '2022-01-01' AND '2022-01-31' ORDER BY `attendances`.`id` DESC

nilの場合でもallメソッドが実行されるのでメソッドチェーンが使える。

class Book
  scope :published, -> { nil }
end

Book.published
#=> nil

Book.published.order(id: :desc)
#=> Attendance Load (3.8ms)  SELECT `attendances`.* FROM `attendances` WHERE `attendances`.`date` BETWEEN '2022-01-01' AND '2022-01-31' ORDER BY `attendances`.`id` DESC

参考