[Solved]-IntegrityError: null value in column "city_id " violates not-null constraint

city = models.OneToOneField(City)

Are you sure you want only one user per city? I think you need

city = models.ForeignKey(City, null=True, blank=True)

null, blank because the sycndb will create a profile and will fail if no city is passed.

Alternatively, you can pass default=default_city_id to the city ForeignKey