Difference between auto_now and auto_now_add
Question:
What I understood in Django models field’s attributes is
auto_now – updates the value of field to current time and date every time the Model.save() is called.auto_now_add – updates the value with the time and date of creation of record.
My question is what if a filed in model contains both the auto_now and auto_now_add set to True? What happens in that case?
Answers:
auto_now takes precedence (obviously, because it updates field each time, while auto_now_add updates on creation only). Here is the code for DateField.pre_save method:
def pre_save(self, model_instance, add):
if self.auto_now or (self.auto_now_add and add):
value = datetime.date.today()
setattr(model_instance, self.attname, value)
return value
else:
return super().pre_save(model_instance, add)
As you can see, if auto_now is set or both auto_now_add is set and the object is new, the field will receive current day.
The same for DateTimeField.pre_save:
def pre_save(self, model_instance, add):
if self.auto_now or (self.auto_now_add and add):
value = timezone.now()
setattr(model_instance, self.attname, value)
return value
else:
return super().pre_save(model_instance, add)
According to the django documentation using both auto_now and auto_now_add in your model fields as True will result in an error because they are both mutually exclusive.
As Official Django documentation says –
auto_now, auto_now_add and default are mutually exclusive and will result in an error if used together
These fields are built into Django for expressly this purpose — auto_now fields are updated to the current timestamp every time an object is saved and are therefore perfect for tracking when an object was last modified, while an auto_now_add field is saved as the current timestamp when a row is first added to the database and is therefore perfect for tracking when it was created.
If a filed in a model contains both the auto_now and auto_now_add set to True will result in an error like this:
ERRORS:
appname.Model.field_name: (fields.Efield_number) The options auto_now, auto_now_add, and default are mutually exclusive. Only one of these options may be present.
What I understood in Django models field’s attributes is
auto_now– updates the value of field to current time and date every time the Model.save() is called.auto_now_add– updates the value with the time and date of creation of record.
My question is what if a filed in model contains both the auto_now and auto_now_add set to True? What happens in that case?
auto_now takes precedence (obviously, because it updates field each time, while auto_now_add updates on creation only). Here is the code for DateField.pre_save method:
def pre_save(self, model_instance, add):
if self.auto_now or (self.auto_now_add and add):
value = datetime.date.today()
setattr(model_instance, self.attname, value)
return value
else:
return super().pre_save(model_instance, add)
As you can see, if auto_now is set or both auto_now_add is set and the object is new, the field will receive current day.
The same for DateTimeField.pre_save:
def pre_save(self, model_instance, add):
if self.auto_now or (self.auto_now_add and add):
value = timezone.now()
setattr(model_instance, self.attname, value)
return value
else:
return super().pre_save(model_instance, add)
According to the django documentation using both auto_now and auto_now_add in your model fields as True will result in an error because they are both mutually exclusive.
As Official Django documentation says –
auto_now, auto_now_add and default are mutually exclusive and will result in an error if used together
These fields are built into Django for expressly this purpose — auto_now fields are updated to the current timestamp every time an object is saved and are therefore perfect for tracking when an object was last modified, while an auto_now_add field is saved as the current timestamp when a row is first added to the database and is therefore perfect for tracking when it was created.
If a filed in a model contains both the auto_now and auto_now_add set to True will result in an error like this:
ERRORS:
appname.Model.field_name: (fields.Efield_number) The options auto_now, auto_now_add, and default are mutually exclusive. Only one of these options may be present.