Django empty field fallback

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.

Although this does not meet all the OP's requirements, in some use-cases where the fallback is needed in queryset filters (on the database level) it may be possible to use Django's Coalesce() class (docs).

The simplest solution in such a case might be queryset annotation (using the example from @daniel-roseman's answer):

queryset = MyModel.objects.annotate(first_name=Coalesce('_first_name', 'user__first_name'))

Each item obtained from the queryset will then get a first_name attribute, which has the value of _first_name with a fallback (in case the value is null) to user.first_name.

That is, assuming your model has as user relation.

This can be implemented e.g. in a custom manager and could also be used in conjunction with a property approach.

A detailed example for a similar case (override instead of fallback) is provided in this SO answer.