Timeslip - trying to limit method to one timeslip per car

gistfile1.txt

class Timeslip < ActiveRecord::Base
  belongs_to :car
  belongs_to :user
  
  has_one :car_model, through: :car
  has_one :transmission, through: :car
  has_one :engine, through: :car
  
  default_scope order('et1320 ASC')
  
  def youtube_id
    regex = /^(?:http:\/\/)?(?:www\.)?\w*\.\w*\/(?:watch\?v=)?((?:p\/)?[\w\-]+)/
    video_url.match(regex)[1]
  end
  
  def self.by_carmodel(carmodel)
    scoped = joins(:car_model).where(:car_models => {:name => carmodel})
    scoped
  end
  
  def self.by_trans(trans)
    scoped = joins(:transmission).where(:transmissions => {:name => trans})
    scoped
  end
  
  def self.one_per_car
    scoped = unscoped.select("car_id").group("car_id")
    scoped
  end
end