Rails User.joins.not（...）在Active Record中？

Question

Im looking to query all Users without Comments in a single sql query?

Models:

class User < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :user
end

So I want the opposite of this:

User.joins.(:comments).group('users.id')

But not like this: (because it generates two queries)

User.where.not(id: Comment.pluck(:user_id))

Maybe something like this?

User.joins.not.(:comments).group('users.id')

Thanks for any input!

Answer 1

You can accomplish this with:

User.includes(:comments).where.not(comments: { id: nil })

This will result in raw SQL that looks something like:

SELECT DISTINCT `users`.`*` FROM `users` LEFT OUTER JOIN `comments` ON `comments`.`user_id` = `users`.`id` WHERE `comments`.`id` IS NULL

For accomplishing this via a subquery, see the below answer.

Old Answer :

You can do something like

User.where.not(id: Comment.select(:user_id))

If you wanted a single (though nested) query.

Otherwise, check out http://guides.rubyonrails.org/active_record_querying.html#joining-tables for using an outer join.

Answer 2

If you are using postgresql you can do something like this

User.joins("LEFT join comments c on users.id = c.comment_id").
select("users.id").
group("users.id").
having("count(users.id) = 1")

This will generate this query

select u.id from users u
LEFT join comments c 
on c.comment_id = u.id
group by u.id
having count(u.id) = 1

This query is not generating two SQL (neither nested sql), the above answer does.

Answer 3

另一个简单的解决方案User.where("id NOT IN (?)", Comment.pluck(:user_id))