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

Question

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.

Answer 1

This is a bug in Rails 4.1. See https://github.com/rails/rails/issues/13648

Answer 2

When you use select and you have more than one columns for an argument, you need to put it in a string, in this format:

User.select("id, email")

In other words, the list of columns should only be passed in pure string format

Answer 3

Thanks @camomileCase for your tip.

In my case I've resolved it by adding count with :all option, and sometimes adding an additional count if needed. This way works perfect for me:

@grid = myscope.select(:thickness, :width, :length, "SUM(quantity_available) as quantity_available").group(:thickness, :width, :length)

@grid.assets.limit(nil).count(:all)
@grid.assets.limit(nil).count(:all).count