[SOLVED] Rails select() and count() don't seem to play nice

Rails select() and count() don't seem to play nice

I've noticed something odd with Rails (4.1) ActiveRecord, where select and count sometimes mix badly:

User.all.count
 => 103
User.all.size
 => 103
User.all.length
 => 103

So far, so good. I can select on id:

User.select(:id).all.count
 => 103
User.select(:id).all.size
 => 103
User.select(:id).all.length
 => 103

Still good. I can also select on email:

User.select(:email).all.count
 => 103
User.select(:email).all.size
 => 103
User.select(:email).all.length
 => 103

But now problems begin. When I select on both id and email:

User.select(:id, :email).all.count
 (0.4ms)  SELECT COUNT(id, email) FROM `users`
Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' email) FROM `users`' at line 1: SELECT COUNT(id, email) FROM `users`
ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' email) FROM `users`' at line 1: SELECT COUNT(id, email) FROM `users`

User.select(:id, :email).all.size
 (0.4ms)  SELECT COUNT(id, email) FROM `users`
Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' email) FROM `users`' at line 1: SELECT COUNT(id, email) FROM `users`
ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' email) FROM `users`' at line 1: SELECT COUNT(id, email) FROM `users`

User.select(:id, :email).all.length
 => 103

Why is it that count and size (which in this case aliases to count) are throwing an exception, and only when I select more than one attribute?

Is there an explanation for this? I find this quite unexpected.

Solution