query , can not select column count

Tag.joins(:quote_tags).group('quote_tags.tag_id').order('count desc').select('count(tags.id) AS count, tags.id, tags.name')

Build query:
SELECT count(tags.id) AS count, tags.id, tags.name FROM `tags` INNER JOIN `quote_tags` ON `quote_tags`.`tag_id` = `tags`.`id` GROUP BY quote_tags.tag_id ORDER BY count desc

Result:

[#<Tag id: 401, name: "different">, ... , #<Tag id: 4, name: "family">]

It not return count column for me. How can I get it?

Have you tried calling the count method on one of the returned Tag objects? Just because inspect doesn't mention the count doesn't mean that it isn't there. The inspect output:

[#<Tag id: 401, name: "different">, ... , #<Tag id: 4, name: "family">]

will only include things that the Tag class knows about and Tag will only know about the columns in the tags table: you only have id and name in the table so that's all you see.

If you do this:

tags = Tag.joins(:quote_tags).group('quote_tags.tag_id').order('count desc').select('count(tags.id) AS count, tags.id, tags.name')

and then look at the counts:

tags.map(&:count)

You'll see the array of counts that you're expecting.

Update: The original version of this answer mistakenly characterized select and subsequent versions ended up effectively repeating the current version of the other answer from @muistooshort. I'm leaving it in it's current state because it has the information about using raw sql. Thanks to @muistooshort for pointing out my error.

Although your query is in fact working as explained by the other answer, you can always execute raw SQL as an alternative.

There are a variety of select_... methods you can choose from, but I would think you'd want to use select_all. Assuming the build query that you implicitly generated was correct, you can just use that, as in:

ActiveRecord::Base.connection.select_all('
  SELECT count(tags.id) AS count, tags.id, tags.name FROM `tags`
    INNER JOIN `quote_tags` ON `quote_tags`.`tag_id` = `tags`.`id`
    GROUP BY quote_tags.tag_id
    ORDER BY count desc')

See http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html for information on the various methods you can choose from.