Add required field during API create

MODELS.PY

class CustomerPackage(models.Model):
    reference = models.CharField(max_length=50, blank=True)
    package_date = models.DateField(auto_now_add=True)
    guests = models.ManyToManyField(Customer, related_name='%(class)s_guests')


class AbstractBaseBooking(models.Model):
    customer = models.ForeignKey(Customer)
    booking_agent = models.ForeignKey(UserProfile)
    venue = models.ForeignKey(Venue)
    duration = models.CharField(max_length=10, choices=DURATION_CHOICES, default='1')
    booked_on = models.DateField(auto_now_add=True)
    booking_start = models.DateTimeField()
    pax = models.IntegerField(verbose_name='Number Of People')
    notes = models.TextField(blank=True, default='')
    customer_package = models.ForeignKey(CustomerPackage)

    class Meta:
        abstract = True
        ordering = ('booking_start',)

    def clean(self):
        assert False
        if self.customer_package is None:
            cp = CustomerPackage()
            cp.save()
            assert False
            self.customer_package = cp

        return super(AbstractBaseBooking, self).clean()


class RestaurantBooking(AbstractBaseBooking):
    table = models.ForeignKey(Table)


VIEWS.PY
class RestaurantBookingListApiView(PreventAPICsrfCheckMixin, generics.ListCreateAPIView):
    '''
    API to list restaurant bookings or create a new one
    '''
    queryset = RestaurantBooking.objects.all()
    serializer_class = RestaurantBookingSerializer

    def pre_save(self, obj):
        assert False


SERIALIZERS.PY
class RestaurantBookingSerializer(serializers.ModelSerializer):
    class Meta:
        model = RestaurantBooking