title: Django admin custom validation - require at least one inline foreign-key model
body: <p>I have two Django models (Purchaser and LineItem) that I manage via the stock admin interface.  The dumbed-down versions:</p>
  
  <pre><code>class Purchaser(models.Model):
      firstname = models.CharField('First Name', max_length = 30)
      lastname = models.CharField('Last Name', max_length = 30)
      paymentid = models.IntegerField('Payment ID', unique = True)
  
  class LineItem(models.Model):
      purchaser = models.ForeignKey(Purchaser)
      ship_first_name = models.CharField('Recipient First Name', max_length = 50)
      ship_last_name = models.CharField('Recipient Last Name', max_length = 50)
  </code></pre>
  
  <p>I have LineItems as an inline within the Purchaser admin page, and want to require that Purchasers have at least one LineItem (i.e. not let the user save a new Purchaser unless they have added at least one LineItem).  Is there a clean way to do this?  I already have some validation set up using a custom <a href="https://stackoverflow.com/questions/3541201/how-can-i-create-sophisticated-django-model-validation-for-django-admin">modelForm</a>, but that method only deals with Purchaser fields, and not anything to do with LineItems.  Advice?</p>