Django South - Create Not Null ForeignKey - django

Question

I have a Model
class Mystery(models.Model):
first = models.CharField(max_length=256)
second = models.CharField(max_length=256)
third = models.CharField(max_length=256)
player = models.ForeignKey(Player)
I added the player ForeignKey but when I try to migrate it using South it seems that I can't create this whith a null=False. I have this message :
The field 'Mystery.player' does not have a default specified, yet is
NOT NULL. Since you are adding this field, you MUST specify a default
value to use for existing rows. Would you like to:
1. Quit now, and add a default to the field in models.py
2. Specify a one-off value to use for existing columns now
I use this command :
manage.py schemamigration myapp --auto
Thanks a lot !

Answer 1

This is best accomplished in 3 migrations.
Step 1. Create the new model and allow player to be null: player = models.ForeignKey(Player, null=True)
Step 2. Run ./manage.py schemamigration <app> --auto
Step 3. Run ./manage.py datamigration <app> set_default_players
Step 4. Update forwards and backwards in <app>/migrations/<number>_set_default_players.py to use whatever logic you like for setting the default player. Ensure that every Mystery object has a value for player.
Step 5. Update the Mystery model so that player has null=False.
Step 6. Run ./manage.py schemamigration <app> --auto
Step 7. Run ./manage.py migrate <app>

Answer 2

Another option is to create a data migration before adding the ForeignKey, in which you create a new Player instance with a specific id. Be sure that that id does not exist previously in your database.
1.Create the data migration file
$ ./manage.py datamigration myapp add_player
Created 00XX_add_player.py
2.Edit the forwards and backwards methods of the file:
def forwards(self, orm):
orm['myapp.Player'].objects.create(name=u'Very misterious player', id=34)
def backwards(self, orm):
# Haven't tested this one yet
orm['myapp.Player'].objects.filter(id=34).delete()
3.Add the ForeignKey to your Mistery class and migrate the schema again. It will ask for the default value to your data migration id, in this example, 34.
$ ./manage.py schemamigration --auto myapp
? The field 'Mistery.player' does not have a default specified, yet is NOT NULL.
? Since you are adding this field, you MUST specify a default
? value to use for existing rows. Would you like to:
? 1. Quit now, and add a default to the field in models.py
? 2. Specify a one-off value to use for existing columns now
? Please select a choice: 2
? Please enter Python code for your one-off default value.
? The datetime module is available, so you can do e.g. datetime.date.today()
>>> 34
+ Added field player on myapp.Mistery
Created 0010_auto__add_field_mistery_player.py. You can now apply this migration with: ./manage.py migrate myapp
4.Finally run the migrate command and it will execute the migrations in sequential order, inserting the new Player and updating all the Mistery rows with the reference to your new Player.

Answer 3

If your database already contains Mystery objects, then south must know, what value put into player field, because it cannot be blank.
One possible solution:
choose
2. Specify a one-off value to use for existing columns now
and then enter 1. So all of your existing Mystery objects will now point to Player with pk = 1. Then you can change (if needed) this in admin page.