How to increment slot field when booking an appointment

Doctor model

class Doctor(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    qualifications = models.CharField(max_length=100)
    expertise = models.CharField(max_length=100, default='Cardiac')
    fee = models.IntegerField(default=500)
    slots = models.IntegerField(default=0)

    def incSlots(self):
        self.slots = self.slots + 1

Appointment model

class Appointment(models.Model):
    appointment_id = models.AutoField(primary_key=True)
    patient = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    blood_pressure = models.CharField(max_length=10)
    BMI = models.CharField(max_length=10)
    blood_sugar = models.CharField(max_length=10)
    HB = models.CharField(max_length=10)
    platelets = models.CharField(max_length=10)
    date = models.CharField(max_length=30)
    time = models.CharField(max_length=30)
    detail = models.TextField()
    doc = models.OneToOneField(Doctor, on_delete=models.CASCADE)

Appointment view

def appointment(request):
    docname = request.POST.get('doc_name')
    if request.method == 'POST':
        form = AppointmentForm(request.POST)
        if form.is_valid():
            form.doc.incSlots()
            form.save()
            return redirect('success')
        else:
            print(form.errors)
    form = AppointmentForm(initial={"patient":request.user, "doc_name": docname})
    return render(request, 'appointment.html', {'form': form})

I want to increment "slot" field when booking an appointment, I have no idea how to do it, i am new to django, i'll appreciate any help Thank you.