问题
I have this model:
class Movie(models.Model):
# here are some fields for this model
to which I added the following field (the database was already populated with Movie models):
user = models.ForeignKey(User, default=1)
I ran the commands 'makemigrations' and then 'migrate':
python manage.py makemigrations myapp
python manage.py migrate
but it doesn't work. What I want to do is to add the 'user' field to Movie objects and provide a default for it for all the existing Movie objects in my database (in this case the default is the User object with id=1).
Another thing that I tried is to leave it without the default value and then, when i run the 'makemigrations' command, to give it the default value 1 (by selecting the "provide a one-off default now" option). In both cases, when I run the 'migrate' command I get an IntegrityError:
django.db.utils.IntegrityError: movies_movie__new.user_id may not be NULL
I also checked the ids for the Users that are already in the database to make sure that a User with id=1 exists and it does. So why doesn't it work? Thank you.
回答1:
If you add "null=True" it works:
user = models.ForeignKey(User, default=1, null=True)
Then
python manage.py makemigrations movies
Migrations for 'movies':
0003_auto_20150316_0959.py:
...
- Add field user to Movie
...
Then
python manage.py migrate movies
The database should look like:
id|user_id|name
1|1|movie_name
2|1|movie_2
3|1|movie_3
...
All the empty user_ids get a value of 1. So you're done.
Now, if you need to make that field required, just change the model back to
user = models.ForeignKey(User, default=1)
make another migration and migrate again
This time it will work since all fields have user_id.
;-)
来源：https://stackoverflow.com/questions/29069306/provide-a-default-for-foreignkey-field-on-existing-entries-in-django