[Active Record]count_all
https://zenn.dev/igaiga/books/rails-practice-note/viewer/ar_usefuls#group%E3%83%A1%E3%82%BD%E3%83%83%E3%83%89%E3%81%A8count%E3%83%A1%E3%82%BD%E3%83%83%E3%83%89 から引用
Book.group(:title).count
#=> SELECT COUNT(*) AS "count_all", "books"."title" AS "books_title" FROM "books" GROUP BY "books"."title"
#=> {"book0"=>2, "book1"=>2, "book2"=>3, "book3"=>1, "book4"=>1}
countメソッドを使うとSQLにcount_allが指定される。 SQLにas xxxを書くと、ActiveRecordがattributesとしてxxxメソッドを作るため、以下のようにcount_allを使った書き方ができる
Book.group(:title).order(count_all: :desc).count
#=> SELECT COUNT(*) AS "count_all", "books"."title" AS "books_title" FROM "books" GROUP BY "books"."title" ORDER BY "count_all" DESC