title: Django admin form - how to change dynamically select values
body: <p>I have that situation:
  I have models Item, Region and Country.</p>
  
  <pre><code>class Item(models.Model):
      name = models.CharField(max_length=255)
      alias = models.SlugField(unique=True)
      country = models.OneToOneField(Country, default=0)
      region = models.OneToOneField(Region, default=0, related_name='')
  
  class Region(models.Model):
      name = models.CharField(max_length=100)
      country = models.ForeignKey(Country, default=0) 
  
  class Country(models.Model):
      name = models.CharField(max_length=100)
  </code></pre>
  
  <p>When I add an item in the admin area and I select country I want to automatically build the region select with <code>Regions</code> only from the selected <code>Country</code>.</p>
  
  <p>I know how to do it in javascript, but i don't know how CORRECT it is do that in Django.</p>