title: Rails: create through-record via association
body: <p>If anyone has a better title, please let me know :p</p>
  
  <p>I have the following models:</p>
  
  <pre><code>class Car
    has_many :car_drivers
    has_many :drivers, :through =&gt; :car_drivers
  end
  
  class Driver
    has_many :car_drivers
    has_many :cars, :through =&gt; :car_drivers
  end
  
  class CarDriver
    belongs_to :car
    belongs_to :driver
  end
  </code></pre>
  
  <p>Now I want to create a new Driver via Car, but the record in the join-table (car_drivers) should be created as well. I tried the following, but while the car record is created, the join-table record is not: <code>driver_object.cars.create</code></p>
  
  <p>What is the best practice in this case?</p>