LEFT OUTER加入Rails 3

Question

I have the following code:

@posts = Post.joins(:user).joins(:blog).select

which is meant to find all posts and return them and the associated users and blogs. However, users are optional which means that the INNER JOIN that :joins generates is not returning lots of records.

How do I use this to generate a LEFT OUTER JOIN instead?

Answer 1

@posts = Post.joins("LEFT OUTER JOIN users ON users.id = posts.user_id").
              joins(:blog).select

Answer 2

You can do with this with includes as documented in the Rails guide :

Post.includes(:comments).where(comments: {visible: true})

Results in:

SELECT "posts"."id" AS t0_r0, ...
       "comments"."updated_at" AS t1_r5
FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"
WHERE (comments.visible = 1)

Answer 3

I'm a big fan of the squeel gem :

Post.joins{user.outer}.joins{blog}

It supports both inner and outer joins, as well as the ability to specify a class/type for polymorphic belongs_to relationships.

Answer 4

使用eager_load ：

@posts = Post.eager_load(:user)

Answer 5

By default when you pass ActiveRecord::Base#joins a named association, it will perform an INNER JOIN. You'll have to pass a string representing your LEFT OUTER JOIN.

From the documentation :

:joins - Either an SQL fragment for additional joins like " LEFT JOIN comments ON comments.post_id = id " (rarely needed), named associations in the same form used for the :include option, which will perform an INNER JOIN on the associated table(s), or an array containing a mixture of both strings and named associations.

If the value is a string, then the records will be returned read-only since they will have attributes that do not correspond to the table's columns. Pass :readonly => false to override.

Answer 6

There is a left_outer_joins method in activerecord. You can use it like this:

@posts = Post.left_outer_joins(:user).joins(:blog).select

Answer 7

Good news, Rails 5 now supports LEFT OUTER JOIN . Your query would now look like:

@posts = Post.left_outer_joins(:user, :blog)

Answer 8

class User < ActiveRecord::Base
     has_many :friends, :foreign_key=>"u_from",:class_name=>"Friend"
end

class Friend < ActiveRecord::Base
     belongs_to :user
end


friends = user.friends.where(:u_req_status=>2).joins("LEFT OUTER JOIN users ON users.u_id = friends.u_to").select("friend_id,u_from,u_to,u_first_name,u_last_name,u_email,u_fbid,u_twtid,u_picture_url,u_quote")