[Answered ]-Django: Populating data into a model based on an identifier

I am doing something similar in my UserProfile model. I have a zip code field that, if it gets filled in by the user, is used to do a geo lookup to get city/state and lat/lng and store them in their respective fields in the model:

class UserProfile(models.Model):
    user = models.ForeignKey(User, unique=True)
    ...
    zip = models.CharField(max_length=12, blank=True, null=True)
    city_state = models.CharField(max_length=30, blank=True, null=True)
    lat = models.DecimalField(max_digits=12, decimal_places=9, blank=True, null=True)
    lng = models.DecimalField(max_digits=12, decimal_places=9, blank=True, null=True)

    def save(self, *args, **kwargs):
        if self.zip:
            (city_state, lat, lng) = get_lat_lng(self.zip)
            if city_state and lat and lng:
                self.city_state = city_state
                self.lat = lat
                self.lng = lng

        super(UserProfile, self).save(*args, **kwargs)

class UserProfileForm(ModelForm):
    class Meta:
        model = UserProfile
        fields = ('zip',)

Note that the derived form’s zip field is the only one visible to the user.