TypeError: __init__() missing 1 required positional argument: 'on_delete'

1.ForeignKey-->on_delete=models.CASCASE(or None,DO_NOTHING,PROTECT,SET_NULL,SET_DEFAULT,SET())

For Example:

 customer = models.ForeignKey("Customer",on_delete=models.CASCADE)

2.OneToOneField--ditto

user = models.OneToOneField(User,on_delete=models.CASCADE)

3.ManyToManyField-->Don't need!