class Car
   include Mongoid::Document
    belongs_to :drivable, polymorphic: true
end

class Driver
   include Mongoid::Document
   has_many :cars, as: :drivable
end

driver = Driver.create
Car.where(drivable: driver).first_or_initialize 

#=> #<Car _id: 5587e09d7661671716010000, drivable_type: nil, drivable_id: BSON::ObjectId('5587e0997661671716000000')>