title: block in assert_valid_keys': Unknown key: :order (ArgumentError)
body: <p>I'm running rails 4.1.0.rc2 and I'm getting the ArgumentError <code>block in assert_valid_keys': Unknown key: :order</code> when I try to do: </p>
  
  <pre><code>has_many :workout_exercises, dependent: :destroy, order: "exercise_order DESC"`
  </code></pre>
  
  <p>I want to put an order constraint on my join model, and as far as I can tell, this worked with Rails 3.2, so I can't figure out what is going on. Any ideas? </p>