为什么当使用'includes'时Rails ActiveRecord会为'.count'生成'count（distinct model.id）'

Question

I am using Rails 3.0.11 with MySQL 5.1 and today I realized that it generates an unexpected SQL statement when calling count on a ActiveRecord::Relation .

More details:

I have the model Profile which belongs to an Account . Assuming that I do the following:

p = Profile.includes(:account).where("accept_threshold >= 0")
p.count

( accept_threshold is an attribute of Profile )

the generated SQL statement is:

SELECT COUNT(DISTINCT `profiles`.`id`) FROM `profiles` LEFT OUTER JOIN `accounts` ON `accounts`.`id` = `profiles`.`account_id` WHERE (accept_threshold >= 0)

This is really a surprise to me. I would expect:

 SELECT COUNT(*) FROM `profiles` LEFT OUTER JOIN `accounts` ON `accounts`.`id` = `profiles`.`account_id` WHERE (accept_threshold >= 0)

On the other hand, the following piece of code:

p = Profile.where("accept_threshold >= 0")
p.count

generates

SELECT COUNT(*) FROM `profiles` WHERE (accept_threshold >= 0)

Do you know why is this the case?

How can I force it to generate COUNT(*) instead of COUNT(DISTINCT `profiles`.`id`) ?

I have tried

p.count(:select => "*") 

but it does not work.

Answer 1

If it didn't do a count distinct it would be count of the number of account records where the accept_threshold is >= 0 .

You are after a count of profile records, not a count of account records.