Django Admin: how to display properties defined on model in an inline?

This is a follow-up to this question. How do I display properties defined on a child model in an inline on the parent? To illustrate, I have this model:

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:
            ret开发者_运维百科urn None

        td = datetime.date.today() - self.birthday
        return td.days / 365

Question is: how do I show 'age' in an inline on User? This is what I have:

class UserProfileInline(admin.StackedInline):
    model = UserProfile
    fk_name = 'user'
    max_num = 1
    fieldsets = [
        ('Demographics', {'fields': ['birthday', 'age']}),
    ]

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:

ImproperlyConfigured: 'UserProfileInline.fieldsets[1][1]['fields']' refers to field 'age' that is missing from the form.

Add the field to both the readonly_fields tuple and fieldsets field as well.

Note this only works in Django 1.2+.