title: use Rails validations to limit to one currently active model
body: <p>Using Rails 3 validations and/or callbacks, what would be the cleanest way to ensure that only one record of a model has a boolean value ticked as true? I'd like to mark one record as the currently active model.</p>
  
  <p>(I know another option is to use a has-one association, but I'm curious to know how to store this more directly in the model records.)</p>