如何使用Rails ActiveRecord进行LEFT OUTER JOIN？

Question

I don't have any ideas. Could you give me any clues (like reference sites). Any help will be appreciated.

Model1: GROUP(id, name)
Model2: USER_GROUP_CMB(id, user_id, group_id)

Expected SQL statement:

SELECT * 
FROM groups AS g LEFT OUTER JOIN user_group_cmbs AS cmb 
            ON g.id = cmb.group_id
WHERE cmb.user_id = 1

I tried to set up associations below but I dont know what to do after this.

class Group < ActiveRecord::Base
  has_many :user_group_cmb
end

class UserGroupCmb < ActiveRecord::Base
  has_many :group
end

Rails Version: 3.1.1

Answer 1

我相信如果您对includes关联使用的表执行条件，则includes将使用LEFT OUTER JOIN查询：

Group.includes(:user_group_cmb).where(user_group_cmbs: { user_id: 1 })

Answer 2

Rails 5+ allows you to do the following:

Group.left_outer_joins(:user_group_cmb)

http://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-left_joins

Answer 3

You can do this in rails 3.x regardless if you are referencing the table or not in a where clause:

Group.eager_load(:user_group_cmb)

...and it will perform a left outter join

Answer 4

It might be quite important to mention that using includes has possibly unwanted side-effects.

if the filtered association is subsequently scoped, all of the original filtering disappears

As it turns out, by scoping on the filtered association we've lost any filtering-as-side-effect that we attained from includes. And it's not because of how we searched, either.

Make sure to read the complete article and alternatively there's a gem for that .

Answer 5

Use a custom joins statement:

Group.joins("left outer join user_group_cmbs as cmb on groups.id = cmb.group_id")
  .where("cmb.user_id = ?", 1)

Answer 6

The problem with the accepted answer is that it will actually do a LEFT INNER JOIN technically because it won't display entries where the user_group_cmbs. user_id user_group_cmbs. user_id is null (which would be the reason to do a LEFT OUTER JOIN ).

A working solution is to use #references :

Group.includes(:user_group_cmb).references(:user_group_cmb)

Or the even more convenient #eager_load :

Group.eager_load(:user_group_cmb)

Read the more detailed explanation here .

Answer 7

Use has_and_belongs_to_many if you just need to link users to groups. Use has_many :through if you need to store additional membership information.

Example:

class User < ActiveRecord::Base
  has_and_belongs_to_many :groups
end

class Group < ActiveRecord::Base
  has_and_belongs_to_many :users      
end

Answer 8

Couldn't comment to an earlier answer because well I don't have 50 reputation. But here's how it worked for me when I ran into below error

includes-referenced relation raises a StatementInvalid exception, complaining it misses a FROM-clause entry. I need to add an explicit call to join, which defaults to being inner.

I had to use .references and mention the table name or for it get added to the FROM-clause