How to add conditional validation in symfony 2.1 forms

Possible duplicate:
Assert property dependent on another property symfony 2

I have an object as shown below:

class Items {

    /*
     * @Assert\NotBlank(message="Title cannot be blank")
    */
    private $title;

    /*
     * @Assert\Range(
     *      min = "0.1",
     *      minMessage = "Price should be a positive number",
     *      invalidMessage = "Price should be in number."
     *  )
     */
     private $price;

     private $currency;

}

      

        
        
        
      

    

I am using symfony 2.1 with a form of ItemType class.

I want to add validation constraints to $ currency only if the price is valid. How can i do this? I want to do this both from the controller and if possible in the form class so that it is useful to others as well, as well