[英]Left outer join using ruby on rails (>=6) ORM?
Rails >= 5 似乎内置了此功能。
这是一个简单的例子:
User.left_outer_joins(:posts)
=> SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
这是另一个例子:
Author.left_joins :posts, :comments
# Produces:
Author Load (0.1ms) SELECT "authors".* FROM "authors" LEFT OUTER JOIN "posts" ON "posts"."author_id" = "authors"."id" LEFT OUTER JOIN "comments" ON "comments"."author_id" = "authors"."id"
这个来自 RailsGuides 的例子也很容易知道(尽管它包含一些原始 SQL):
Customer.left_outer_joins(:reviews).distinct.select('customers.*, COUNT(reviews.*) AS reviews_count').group('customers.id')
#Produces:
SELECT DISTINCT customers.*, COUNT(reviews.*) AS reviews_count FROM customers
LEFT OUTER JOIN reviews ON reviews.customer_id = customers.id GROUP BY customers.id
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