Django empty field fallback

Question

Hi there,

I have a model that holds user address. This model has to have first_name and last_name fields since one would like to set address to a recipient (like his company etc.). What I'm trying to achieve is:

• if the fist_name/last_name field in the address is filled - return simply that field
• if the fist_name/last_name field in the address is empty - fetch corrresponding field data from a foreignkey pointing to a proper django.auth.models.User
• I'd like this to be treated as normal django field that would be present in fields lookup
• I don't want to create a method, since it's a refactoring and Address.first_name/last_name are used in various places of the application (also in model forms etc.), so I need this to me as smooth as possible or else I'll have to tinker around in a lot of places

Thanks for help :)

Answer 1

There are two options here. The first is to create a method to look it up dynamically, but use the property decorator so that other code can still use straight attribute access.

class MyModel(models.Model):
    _first_name = models.CharField(max_length=100, db_column='first_name')

    @property
    def first_name(self):
        return self._first_name or self.user.first_name

    @first_name.setter
    def first_name(self, value):
       self._first_name = value

This will always refer to the latest value of first_name, even if the related User is changed. You can get/set the property exactly as you would an attribute: myinstance.first_name = 'daniel'

The other option is to override the model's save() method so that it does the lookup when you save:

def save(self, *args, **kwargs):
    if not self.first_name:
        self.first_name = self.user.first_name
    # now call the default save() method
    super(MyModel, self).save(*args, **kwargs)

This way you don't have to change your db, but it is only refreshed on save - so if the related User object is changed but this object isn't, it will refer to the old User value.