Railsドキュメント

複数のテーブルを結合して取得

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説明

複数のテーブルを結合して取得

使い方

モデル.joins(条件)

categoryテーブルにpostテーブルを結合して取得

Category.joins(:posts)
# SELECT categories.* FROM categories INNER JOIN posts ON posts.category_id = categories.id

2個のテーブルをjoin

Post.joins(:category, :comments)
# SELECT posts.* FROM posts
# INNER JOIN categories ON posts.category_id = categories.id
# INNER JOIN comments ON comments.post_id = posts.id

ネストして結合

Page.joins(categories: :element)
#  SELECT "pages".* FROM "pages" INNER JOIN "pages_categories" ON "pages_categories"."page_id" = "pages"."id" INNER JOIN "categories" ON "categories"."id" = "pages_categories"."category_id" INNER JOIN "elements" ON "elements"."id" = "categories"."element_id"

SQLで記述

User.joins("LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id")
# SELECT "users".* FROM "users" LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id

ソースコード