【Rails】scopeの注意点

scopeのメリット

• 繰り返し利用するクエリの再利用性が上がる
• クエリに名前を付けることで、可読性が向上する

scopeの注意点

scopeの結果がnilとなった場合は、nilを返す該当scopeの検索条件を除外したクエリを発行し、必ずActiveRecord::Relationを返すという動作が行われる

本記事は、こちらの注意点にフォーカスします。

（先に）結論

nilを返す可能性がある場合にはscopeで定義せず、クラスメソッドで定義する

動作確認のための準備

環境

• Rubyバージョン：2.6.6
• Railsバージョン：6.0.3

モデルの作成

rails g model Book book_title:string publication_date:date price:integer

マイグレーション

rails db:migrate

scopeを定義

app/models/book.rb

class Book < ApplicationRecord
  scope :search_by_title, ->(book_title) { where('book_title like ?', "%#{book_title}%") } # 通常のscope
  scope :find_by_price, ->(price) { find_by(price: price) } # nilを返す可能性のあるscope
end

任意のレコードを作成

rails c

(base) sample@SamplenoMBP practice % rails c
Loading development environment (Rails 6.0.3)
irb(main):001:0> (1..5).each { |i| Book.create(book_title: "Book #{i}", publication_date: Time.parse('20240901').ago(i.months), price: (i * 1000)) }
   (0.8ms)  SELECT sqlite_version(*)
   (0.0ms)  begin transaction
  Book Create (1.0ms)  INSERT INTO "books" ("book_title", "publication_date", "price", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["book_title", "Book 1"], ["publication_date", "2024-08-01"], ["price", 1000], ["created_at", "2024-08-31 09:10:12.130910"], ["updated_at", "2024-08-31 09:10:12.130910"]]
   (0.6ms)  commit transaction
   (0.0ms)  begin transaction
  Book Create (0.2ms)  INSERT INTO "books" ("book_title", "publication_date", "price", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["book_title", "Book 2"], ["publication_date", "2024-07-01"], ["price", 2000], ["created_at", "2024-08-31 09:10:12.138869"], ["updated_at", "2024-08-31 09:10:12.138869"]]
   (0.3ms)  commit transaction
   (0.0ms)  begin transaction
  Book Create (0.2ms)  INSERT INTO "books" ("book_title", "publication_date", "price", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["book_title", "Book 3"], ["publication_date", "2024-06-01"], ["price", 3000], ["created_at", "2024-08-31 09:10:12.139852"], ["updated_at", "2024-08-31 09:10:12.139852"]]
   (0.3ms)  commit transaction
   (0.0ms)  begin transaction
  Book Create (0.2ms)  INSERT INTO "books" ("book_title", "publication_date", "price", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["book_title", "Book 4"], ["publication_date", "2024-05-01"], ["price", 4000], ["created_at", "2024-08-31 09:10:12.140731"], ["updated_at", "2024-08-31 09:10:12.140731"]]
   (0.3ms)  commit transaction
   (0.0ms)  begin transaction
  Book Create (0.2ms)  INSERT INTO "books" ("book_title", "publication_date", "price", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["book_title", "Book 5"], ["publication_date", "2024-04-01"], ["price", 5000], ["created_at", "2024-08-31 09:10:12.141575"], ["updated_at", "2024-08-31 09:10:12.141575"]]
   (0.3ms)  commit transaction

敢えてnilが返る条件でscopeを実行

nilを返す(可能性のある)scopeのみの場合

rails c

irb(main):001:0> Book.find_by_price(10000)
  Book Load (0.3ms)  SELECT "books".* FROM "books" WHERE "books"."price" = ? LIMIT ?  [["price", 10000], ["LIMIT", 1]]
  Book Load (0.1ms)  SELECT "books".* FROM "books" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Book id: 1, book_title: "Book 1", publication_date: "2024-08-01", price: 1000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">, #<Book id: 2, book_title: "Book 2", publication_date: "2024-07-01", price: 2000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">, #<Book id: 3, book_title: "Book 3", publication_date: "2024-06-01", price: 3000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">, #<Book id: 4, book_title: "Book 4", publication_date: "2024-05-01", price: 4000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">, #<Book id: 5, book_title: "Book 5", publication_date: "2024-04-01", price: 5000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">]>

該当scopeの条件によって、取得された結果が本来nilとなる場合
→nilになる該当scopeの条件を除いたクエリが発行される

該当scopeの条件を除いた場合、このケースだとBook.allと同等になる為、全てのレコードが返る

通常のscope + nilを返す可能性のあるscope

rails c

irb(main):002:0> Book.search_by_title('Book 1').find_by_price(10000)
    (1.3ms)  SELECT sqlite_version(*)
   Book Load (0.3ms)  SELECT "books".* FROM "books" WHERE (book_title like '%Book 1%') AND "books"."price" = ? LIMIT ?  [["price", 10000], ["LIMIT", 1]]
   Book Load (0.1ms)  SELECT "books".* FROM "books" WHERE (book_title like '%Book 1%') LIMIT ?  [["LIMIT", 11]]
 => #<ActiveRecord::Relation [#<Book id: 1, book_title: "Book 1", publication_date: "2024-08-01", price: 1000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">]>

該当scopeの条件を除いた、
SELECT books.* FROM books WHERE (book_title like '%Book 1%')
の結果（今回の場合だと1件）が返る

本来nilを期待しているとすると、想定しない結果が返ることになる

通常scope(検索結果なし) + nilを返すscopeの場合

rails c

irb(main):004:0> Book.search_by_title('zzz').find_by_price(10000)
  Book Load (0.7ms)  SELECT "books".* FROM "books" WHERE (book_title like '%zz%') AND "books"."price" = ? LIMIT ?  [["price", 10000], ["LIMIT", 1]]
  Book Load (0.3ms)  SELECT "books".* FROM "books" WHERE (book_title like '%zz%') LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation []>

該当のscopeの条件を除いた、
SELECT books.* FROM books WHERE (book_title like '%zz%')
が実行される。

ActiveRecordに対してQuery Interfaceを呼び出しているため、
通常通り「空のActiceRecord::Relation」インスタンスが返る

nilを返す可能性のあるscopeが、nilを返さない場合

想定通り動作する

rails c

irb(main):003:0> Book.where(id: [1,2,3]).find_by_price(2000)
  Book Load (1.0ms)  SELECT "books".* FROM "books" WHERE "books"."id" IN (?, ?, ?) AND "books"."price" = ? LIMIT ?  [["id", 1], ["id", 2], ["id", 3], ["price", 2000], ["LIMIT", 1]]
=> #<Book id: 2, book_title: "Book 2", publication_date: "2024-07-01", price: 2000, created_at: "2024-08-31 09:10:12", updated_at: "2024-08-31 09:10:12">

nilを返さない場合は、想定通りの結果を返す

しかし、nilが返る可能性がある時点でscopeとして定義するのは良くない

nilが想定される場合は、クラスメソッドで定義する

app/models/book.rb

class Book < ApplicationRecord
  def self.find_by_price(price)
    find_by(price: price)
  end
end

rails c

irb(main):001:0> Book.find_by_price(10000)
  Book Load (0.4ms)  SELECT "books".* FROM "books" WHERE "books"."price" = ? LIMIT ?  [["price", 10000], ["LIMIT", 1]]
=> nil

想定通り、nilが返却される