Mongoid with rails, how to do order_by on association count?

I have two models say: user and post

user.rb

has_many :posts

And post.rb

belongs_to :user, :index => true

I need to show users order_by number of posts added by them. One way to do this is: User.all.sort{|a,b| b.posts.count <=> a.posts.count} But this is one of expensive operation.

Also I need a user with highest number of post. So having code like User.all.sort{|a,b| b.posts.count <=> a.posts.count}.first is again another expensive operation.

Is there any other way to achieve this?