How to validate if a string is json in a Rails model

I'm building a simple app and want to be able to store json strings in a db. I have a table Interface with a column json, and I want my rails model to validate the value of the string. So something like:

class Interface < ActiveRecord::Base
  attr_accessible :name, :json

  validates :name,  :presence => true,
                    :length   => { :minimum => 3,
                                   :maximum => 40 },
                    :uniqueness => true

  validates :json, :presence => true,
                   :type => json #SOMETHING LIKE THIS
                   :contains => json #OR THIS    
end

How do I do that?