Rails SQL COUNT N+1 inefficiency

You can use counter cache: http://guides.rubyonrails.org/association_basics.html#counter_cache

"With this declaration, Rails will keep the cache value up to date, and then return that value in response to the size method."

class BlogPost < ActiveRecord::Base
  has_many :blog_comments
end

class BlogComment < ActiveRecord::Base
  belongs_to :blog_post, :counter_cache => true
end

Blog post would have a column named blog_comments_count.

In general, you want a SQL query like:

  SELECT COUNT(*), blog_post_id
    FROM blog_comments
GROUP BY blog_post_id;

You can use this to create a hash from blog_post_id to the count of comments.

You can also see something like this:

BlogComment.group('blog_post_id').count

In purely Rails Way. :)