Rails select（）和count（）看起来玩起来不太好

Question

[我注意到Rails（4.1）ActiveRecord有点奇怪，其中select和count有时混合得不好：

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

到目前为止，很好。我可以选择id：

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

仍然很好。我也可以选择email：

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

但是现在问题开始了。当我在[[both id和email：]]上选择时

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

为什么count和size（在这种情况下为count的别名）引发异常，并且仅当我选择
多个]属性时]

对此有解释吗？我觉得这很意外。

我注意到Rails（4.1）ActiveRecord有点奇怪，其中选择和计数有时会混合在一起：User.all.count => 103 User.all.size => 103 User.all.length => 103到目前为止，这么好。我...

Answer 1

这是Rails 4.1中的错误。参见https://github.com/rails/rails/issues/13648

Answer 2

当您使用select并且一个参数有多个列时，您需要将其以这种格式放入字符串中：

Answer 3

感谢@camomileCase给您的提示。