Rails 3 validates_format_of not working for a simple regex

Having troub开发者_如何转开发le getting a simple validation to work in Rails 3, even though the regex works fine. Even with a valid format, the validation fails.

Model:

validates_format_of :start,
  with:    /\A[0-9]{2}\/[0-9]{2}\/[0-9]{4}\z/,
  message: "Invalid date format"

Assertion:

Event.new(start: "12/10/2010").valid?.should eq(true)

That assertion is failing because valid? returns false.

What am I missing?

The regex fails because start is a Date type, and when the object is instantiated with a string, ActiveRecord automagically converts it to a Date. By the time the validator gets ahold of it, it's no longer a string, so there's no match.