Question

I have a a Comments table and a Posts table, where a Post has many comments and a comment belongs to a post (i.e. it has a post_id in the table).

How can I efficiently select the last ten Posts that DO NOT have comments.
I can’t seem to accomplish this without first selecting all the posts and checking each for a 0 comment count.

Thanks in advance for any help.

-Nathan

Answer 1

You can add a counter cache to the Comment model and a couple of named scopes to the Post model. Something like:

class Post < ActiveRecord::Base
  has_many :comments

  named_scope :recent, :limit => 10, :order => 'created_at DESC'
  named_scope :uncommented, :conditions => { :comments_count => 0 }
end

class Comment < ActiveRecord::Base
  belongs_to :post, :counter_cache => true
end

If you’re using Rails 3 then the named scope syntax is a little different:

class Post < ActiveRecord::Base
  has_many :comments

  scope :recent, limit(10).order('posts.created_at DESC')
  scope :uncommented, where(:comments_count => 0)
end

Now you can find posts without any comments by chaining the named scopes together:

@uncommented = Post.recent.uncommented

If you wanted to get all the posts without any comments (i.e. not just the ten most recent posts), then it would be:

@uncommented = Post.uncommented

—You may have noticed that in the Rails 3 example I included the posts table name in the :recent scope. This is a good practice to get in to so that you avoid ambiguous column names in SQL queries if two tables have the same column name. This Railscast explains more.