Provide a default profile picture if ever empty

I would like to provide a default profile picture for a user. I've used the default feature on the ImageField, but it acts more like an 'initial' than a default. I've read over the answers to this question: Default image for ImageField in Django's ORM, but I'd like to solve the problem at the model level and it does not address with FK relationships.

Here is what I currently have:

class Avatar(models.Model):
    avatar = models.ImageField(upload_to=upload_to)
    avatar_thumbnail = models.ImageField(upload_to=upload_to)
    profile = models.ForeignKey(UserProfile)

class UserProfile(models.Model):
    user = models.ForeignKey(User, unique=True)
    ...

I've tried adding the following function to the UserProfile model (and more than likely, incorrect code) --

 # in UserProfile(models.Model) 
   def create_default_avatar(self):
       if not 开发者_高级运维self.avatar_set.all():
          avatar = profile.avatar_set.create(profile=self.__class__)
          avatar.avatar = File(open('media/default_profile_picture.jpg'))
          avatar.avatar_thumbnail = File(open('media/default_profile_picture_thumbnail.jpg'))
          avatar.save()

Could someone please show me how to create a true default picture for a profile. Thank you.

Ignacio's answer is simple but not perfect. It breaks the DRY principle. You have to add in all your templates the {% if image %} statements. I suggest creating a property on on the model, which will handle that for you (in all places at once).

class Avatar(models.Model):
    (...)

    @property
    def avatar_image_url(self):
        # Pseudocode:
        if has image:
            return url to the image
        else:
            return url to default image

Then in template you just do:

<img src="{{ item.avatar_image_url }}" alt="..."/>

Store nothing in the field if the default is to be used, and use the {% if %} template tag to detect this.