mongoid-example.rb

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title, :details, :author
  # Will run n Mongo queries for n posts being rendered.
  def author
    User.find(object.author_id)
  end
end

# This is now a Mongoid document, not an ActiveRecord model.
class User  
  include Mongoid::Document  
  include Mongoid::Timestamps  
  # ...
end