class Movie
  include Mongoid::Document
  has_many :ratings, as: :ratable
end

class Rating
  include Mongoid::Document
  belongs_to :ratable, polymorphic: true
end

Rating.where(ratable_id: "56cf07ccf1286e1286530b03")
=> #<Mongoid::Criteria
  selector: {"ratable_id"=>"56cf07ccf1286e1286530b03"}
  options:  {}
  class:    Rating
  embedded: false>

Rating.where(ratable_id: "56cf07ccf1286e1286530b03")
=> #<Mongoid::Criteria
  selector: {"ratable_id"=>BSON::ObjectId('56cf07ccf1286e1286530b03')}
  options:  {}
  class:    Rating
  embedded: false>