title: rails f.submit checking another table
body: <p>I understand that in form_for, I'm able to validate that form's table in my model, but what if I wanted to validate the "f.submit"ted data against another table?</p>
  
  <p>Specifically, a Staff can only checkout a Radio in the Checkout model/table if both the Staff and Radio exists in their own respective tables.</p>
  
  <p>How would I go about passing in the Staff and Radio table into my Checkout validation criteria?</p>
  
  <p>Thanks!</p>