Normalize Django Field's Value - django

Question

I wrote a custom django field to normalize the urls our system received. However, the url will only return normalized value after reload.
from django.db import models
def _rewrite_internal_url(url):
#
return 'http://www.google.com/1.jpg'
class NormalizedURLField(models.URLField):
def to_python(self, value):
value = super().to_python(value)
return _rewrite_internal_url(value)
def from_db_value(self, value, expression, connection):
if value is None:
return value
return _rewrite_internal_url(value)
class DjangoTest(models.Model):
url = NormalizedURLField()
instance = DjangoTest.objects.create(url="http://www.google.com/2.jpg")
print(instance.url) # still http://www.google.com/2.jpg
instance.referesh_from_db()
print(instance.url) # update to http://www.google.com/1.jpg

Answer 1

If you do not have non-normalized urls saved in the NormalizedURLField fields, overriding to_python and from_db_value is unnecessary. Instead override pre_save to transform and update the value on the model instance when saving:
class NormalizedURLField(models.URLField):
def pre_save(self, model_instance, add):
attr = super().pre_save(model_instance, add)
rewritten = _rewrite_internal_url(attr)
# update the value on the model instance
setattr(model_instance, self.attname, rewritten)
# return the rewritten value
return rewritten
If you already have non-normalized values in your db that you want converted when fetching from the database then you will still want to implement from_db_value as you have done.