How can I do a LEFT OUTER JOIN using Rails ActiveRecord?

Question

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

Model1: GROUP         


        

Answer 1

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 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

I believe an includes will use a LEFT OUTER JOIN query if you do a condition on the table that the includes association uses:

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

Answer 4

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 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

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