Django DateTimeField with auto_now_add asks for default

I have this field in my model created_at = models.DateTimeField( auto_now_add = True )

When I try to make migrations I get an error:

You are trying to add the field 'created_at' with 'auto_now_add=True' to user wi
thout a default; the database needs something to populate existing rows.

 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py

I tried to set the default value but it says default and auto_now_add are mutually exclusive. Of course I could just use default without auto_now_add but I want to know why this error pops up. Did I miss something?

You have already some rows without created_at value. But when you added following field and run migration

created_at = models.DateTimeField( auto_now_add = True)

It expects the existing row has created_at value. So, It warns you at the time of migrations.

To solve this problem. You can choose the 1st option which says Provide a one-off default now (will be set on all existing rows). By choosing the 1st option you are asking for value. Here you can enter timezone.now(). It will populate existing rows with the current timestamp.

Delete the migration file in app and you can makemigrtion process at the time no error raises, It raises due to the reason of previously created time object.