title: Django Admin: how to display properties defined on model in an inline?
body: <p>This is a follow-up to this <a href="https://stackoverflow.com/questions/3409970/django-admin-how-to-display-fields-from-two-different-models-in-same-view">question</a>. How do I display properties defined on a child model in an inline on the parent? To illustrate, I have this model:</p>
  
  <pre><code>class UserProfile(models.Model):
      user = models.ForeignKey(User, unique=True, primary_key=True, related_name='profile')
      ...
      @property
      def age(self):
          if self.birthday is None:
              return None
  
          td = datetime.date.today() - self.birthday
          return td.days / 365
  </code></pre>
  
  <p>Question is: how do I show 'age' in an inline on User? This is what I have:</p>
  
  <pre><code>class UserProfileInline(admin.StackedInline):
      model = UserProfile
      fk_name = 'user'
      max_num = 1
      fieldsets = [
          ('Demographics', {'fields': ['birthday', 'age']}),
      ]
  </code></pre>
  
  <p>I've tried a few things like this, including 'age()', defining a 'get_age' getter for the Inline, etc. They result in some version of this error:</p>
  
  <pre><code>ImproperlyConfigured: 'UserProfileInline.fieldsets[1][1]['fields']' refers to field 'age' that is missing from the form.
  </code></pre>