Question

Possible Duplicate:
How would I grab only articles with comments that were created 20 minutes ago?

Using mongodb and mongoid. How would one grab all articles and order by number of comments?

class Article
  include Mongoid::Document

  has_many :comments
end

class Comment
  include Mongoid::Document

  belongs_to :article
end

Answer 1

I wasn’t sure on the direction you were intending to sort in, so I included indexes for both – you should delete one, if you’re not planning to use it, but this should do the trick for you:

class Article
  include Mongoid::Document

  field :comments_count, :type => Integer, :default => 0
  index [[ :comments_count, Mongo::ASCENDING ]]
  index [[ :comments_count, Mongo::DESCENDING ]]
  has_many :comments

  before_save :update_comments_count

  protected
  def update_comments_count
    self.comments_count = self.comments.count
  end
end