includesしたのにN+1問題が回避できない

userモデルとtaskモデルに関して、１対多のアソシエーションを組んでおり、

以下のようにビューでeach構文の中でアソシエーションを使ってuserのtasksの数を表示させておりました。

app/views/admin/users/index.html.slim

div.fontsize_midium.margintop_big
  | 管理画面：ユーザー一覧
  span.span_margin_big.btn.light-blue.accent-2
    = link_to "ユーザー新規作成",  new_admin_user_path

- @users.each do |user|
  div.margintop_big.card.fontsize_small.padding_midium
    | ID:
    = user.id
    span.span_margin
    = user.name
    span.span_margin
    = user.created_at
    span.span_margin
    | タスク数:
    = user.tasks.count
    span.span_margin
    =link_to "編集", "/admin/users/#{user.id}/edit"
    span.span_margin
    = link_to "削除", "/admin/users/#{user.id}", method: :delete

N+1問題を防ぐために、コントローラーでincludes(:tasks)としておりました。

app/controllers/admin/users_controller.rb

def index
  @users = User.all.order("created_at DESC").includes(:tasks)
end

しかし、ログを見てみると、N+1問題が発生してしまっておりました。。。

Started GET "/admin/users" for 127.0.0.1 at 2019-02-12 11:44:26 +0900
Processing by Admin::UsersController#index as HTML
  User Load (0.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 3], ["LIMIT", 1]]
  ↳ app/helpers/sessions_helper.rb:8
  Rendering admin/users/index.html.slim within layouts/application
  User Load (0.3ms)  SELECT "users".* FROM "users" ORDER BY created_at DESC
  ↳ app/views/admin/users/index.html.slim:6
  Task Load (0.2ms)  SELECT "tasks".* FROM "tasks" WHERE "tasks"."user_id" IN ($1, $2, $3)  [["user_id", 3], ["user_id", 2], ["user_id", 1]]
  ↳ app/views/admin/users/index.html.slim:6
   (0.3ms)  SELECT COUNT(*) FROM "tasks" WHERE "tasks"."user_id" = $1  "user_id", 3
  ↳ app/views/admin/users/index.html.slim:16
   (0.2ms)  SELECT COUNT(*) FROM "tasks" WHERE "tasks"."user_id" = $1  "user_id", 2
  ↳ app/views/admin/users/index.html.slim:16
   (0.2ms)  SELECT COUNT(*) FROM "tasks" WHERE "tasks"."user_id" = $1  "user_id", 1
  ↳ app/views/admin/users/index.html.slim:16
  Rendered admin/users/index.html.slim within layouts/application (7.3ms)
Completed 200 OK in 45ms (Views: 39.5ms | ActiveRecord: 1.6ms)

解決方法

user.tasks.countとしていた部分を、user.tasks.sizeと変更しました。

app/views/admin/users/index.html.slim

div.fontsize_midium.margintop_big
  | 管理画面：ユーザー一覧
  span.span_margin_big.btn.light-blue.accent-2
    = link_to "ユーザー新規作成",  new_admin_user_path

- @users.each do |user|
  div.margintop_big.card.fontsize_small.padding_midium
    | ID:
    = user.id
    span.span_margin
    = user.name
    span.span_margin
    = user.created_at
    span.span_margin
    | タスク数:
    = user.tasks.size   #countをやめてsizeにした
    span.span_margin
    =link_to "編集", "/admin/users/#{user.id}/edit"
    span.span_margin
    = link_to "削除", "/admin/users/#{user.id}", method: :delete

すると、以下のように、N+1問題も解決しました！！

Started GET "/admin/users" for 127.0.0.1 at 2019-02-12 11:46:38 +0900
Processing by Admin::UsersController#index as HTML
  User Load (0.4ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 3], ["LIMIT", 1]]
  ↳ app/helpers/sessions_helper.rb:8
  Rendering admin/users/index.html.slim within layouts/application
  User Load (0.2ms)  SELECT "users".* FROM "users" ORDER BY created_at DESC
  ↳ app/views/admin/users/index.html.slim:6
  Task Load (0.2ms)  SELECT "tasks".* FROM "tasks" WHERE "tasks"."user_id" IN ($1, $2, $3)  [["user_id", 3], ["user_id", 2], ["user_id", 1]]
  ↳ app/views/admin/users/index.html.slim:6
  Rendered admin/users/index.html.slim within layouts/application (6.4ms)
Completed 200 OK in 38ms (Views: 34.2ms | ActiveRecord: 0.9ms)