Rails has_many association count child rows

Rails 3 Version

For Rails 3, you'd be looking at something like this:

Article.select("articles.*, count(comments.id) AS comments_count")
  .joins("LEFT OUTER JOIN comments ON comments.article_id = articles.id")
  .group("articles.id")

Thanks to Gdeglin for the Rails 2 version.

Rails 5 Version

Since Rails 5 there is left_outer_joins so you can simplify to:

Article.select("articles.*, count(comments.id) AS comments_count")
  .left_outer_joins(:comments)
  .group("articles.id")

And because you were asking about the Rails Way: There isn't a way to simplify/railsify this more with ActiveRecord.

A simple way that I used to solve this problem was

In my model I did:

class Article < ActiveRecord::Base
  has_many :posts

  def count_posts
    Post.where(:article_id => self.id).count
  end
end

Now, you can use for example:

Articles.first.count_posts

Im not sure if it can be more efficient way, But its a solution and in my opinion more elegant than the others.

From a SQL perspective, this looks trivial - Just write up a new query.

From a Rails perspective, The values you mention are computed values. So if you use find_by_sql, the Model class would not know about the computed fields and hence would return the computed values as strings even if you manage to translate the query into Rails speak. See linked question below.
The general drift (from the responses I got to that question) was to have a separate class be responsible for the rollup / computing the desired values.

How to get rails to return SUM(columnName) attributes with right datatype instead of a string?

This activerecord call should do what you want:

Article.find(:all, :select => 'articles.*, count(posts.id) as post_count',
             :joins => 'left outer join posts on posts.article_id = articles.id',
             :group => 'articles.id'
            )

This will return a list of article objects, each of which has the method post_count on it that contains the number of posts on the article as a string.

The method executes sql similar to the following:

SELECT articles.*, count(posts.id) AS post_count
FROM `articles`
LEFT OUTER JOIN posts ON posts.article_id = articles.id
GROUP BY articles.id

If you're curious, this is a sample of the MySQL results you might see from running such a query:

+----+----------------+------------+
| id | text           | post_count |
+----+----------------+------------+
|  1 | TEXT TEXT TEXT |          1 |
|  2 | TEXT TEXT TEXT |          3 |
|  3 | TEXT TEXT TEXT |          0 |
+----+----------------+------------+