在 Rails 中计算关联记录

Question

I am trying to get the number of comments from each of the posts in the database. The following, however:

Post.includes(:comments).group("posts.id").count("comments.id")

raises the mysql error "Unknown column comments.id", since the generated sql seems to completely ignore the includes():

SELECT COUNT(comments.id) AS count_comments_id, posts.id AS posts_id
FROM `posts` GROUP BY posts.id

Interestingly, replacing includes() with joins() will produce working sql:

Post.joins(:comments).group("posts.id").count("comments.id")

SELECT COUNT(comments.id) AS count_comments_id, posts.id AS posts_id
FROM `posts` INNER JOIN `comments` ON `comments`.`post_id` = `posts`.`id`
GROUP BY posts.id

but the above query excludes all posts with 0 comments, which is not what I want. What I do need is to produce the following SQL (but without writing SQL, he he he)

SELECT COUNT(comments.id) AS count_comments_id, posts.id AS posts_id
FROM `posts` LEFT OUTER JOIN `comments` ON `comments`.`post_id` = `posts`.`id`
GROUP BY posts.id

Answer 1

The includes method will not do a join in all cases but rather batch-fetch the association for performance reasons (see Rails :include vs. :joins ).

What you need to do is a joins and you where almost on the correct path but got the group clause a bit wrong:

Post.select("posts.*, COUNT(comments.id) as comment_count").joins("LEFT OUTER JOIN comments ON (comments.post_id = posts.id)").group("posts.id")

Note that this solution has the benefit or actually returning Post objects (using .count() returns a Hash on my Rails 3.2) so you can loop through actual post objects in your view and also access the property comment_count .

Answer 2

This problem is actually way simpler nowadays:

Comment.joins(:post).group(:post_id).count(:post_id)

This will give you a map of {<post_id> => <count of comments>}

Answer 3

Try this:

Post.select("COUNT(comments.id) AS count_comments_id, posts.id AS posts_id").
  includes(:comments).group("posts.id")

Answer 4

If all you need is the hash of post counts: {post_id1 => cnt1, post_id2 => cnt2, ...} then forcing a left join on the association will work:

  Post.joins("LEFT OUTER JOIN comments on posts.id=comments.post_id").
  group("posts.id").count("comments.id")

Answer 5

For rails 5 or above , we have a activerecord method left_outer_joins & we don't need to write the join logic manually.

Post.left_outer_joins(:comments)
    .distinct
    .select('posts.*, COUNT(comments.*) AS comments_count')
    .group('posts.id')

Which produces:

SELECT DISTINCT posts.*, COUNT(comments.*) AS comments_count FROM posts 
LEFT OUTER JOIN comments ON comments.post_id = posts.id GROUP BY posts.id