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I do not think you can have a primary key based on multiple column (cf. django doc : https://code.djangoproject.com/wiki/MultipleColumnPrimaryKeys).
Only unique_together will still work.
In case what you want is an auto incremented field which is not a primary key :
AutoField¶
class AutoField(**options)¶ An IntegerField that automatically
increments according to available IDs. You usually won’t need to use
this directly; a primary key field will automatically be added to your
model if you don’t specify otherwise. See Automatic primary key
fields.
Source:stackexchange.com