title: Django validate one model field against another in admin
body: <p>Say a model has two DateTimeFields:</p>
  
  <pre><code>class Tourney(models.Model):
      registration_deadline = models.DateTimeField()
      start_date = models.DateTimeField()
  </code></pre>
  
  <p>When a user tries to submit a Tourney from within the Django admin, how can I test that <code>registration_deadline</code> is before <code>start_date</code> before saving to the database and, if there is an error, of course inform the user inline, just as Django would if there were any OTHER validation errors?</p>
  
  <p>Basically, I'm looking for custom admin validation.<a href="https://docs.djangoproject.com/en/1.6/ref/forms/validation/#cleaning-a-specific-field-attribute" rel="nofollow">This</a> part of the Django docs is close, but seems to be for forms.  How can I do the 'Cleaning and validating fields that depend on each other' from within Django's admin?  Even just a pointer to the right place in the docs would be sufficient.</p>
  
  <p>Edit: I'm thinking it has something to do with <a href="https://docs.djangoproject.com/en/1.6/ref/validators/" rel="nofollow">validators</a>, but they seem only able to test a single value, not two at the same time...</p>