Update existing M2M relationship in Django - django

Question

I'm trying to save an existing instance of a customer record. Its model has a M2M to the vehicle model (since a customer can multiple vehicles). After reading several questions/answer here, I still do not know how to solve this.
Customer model:
class Customer(models.Model):
vehicle_id = models.ManyToManyField(VehicleSale)
name = models.CharField(max_length=40, blank=True, db_index=True, null=True,
verbose_name='name')
lic = models.CharField(max_length=20, blank=True, db_index=True, null=True,
verbose_name='license')
addr = models.CharField(max_length=40, blank=True, null=True, verbose_name='address')
city = models.CharField(max_length=15, blank=True, null=True, verbose_name='city')
state = models.CharField(max_length=2, blank=True, null=True, verbose_name='state')
zip = models.CharField(max_length=10, blank=True, null=True, verbose_name='zipcode')
email = models.EmailField(blank=True, null=True, verbose_name='email')
tel1 = models.CharField(max_length=15, blank=True, verbose_name='Tel. 1', null=True)
tel2 = models.CharField(max_length=15, blank=True, verbose_name='Tel. 2', null=True)
ssn = models.CharField(max_length=12, blank=True, db_index=True, null=True,verbose_name='SSN')
class Meta:
db_table = 'customer'
def __unicode__(self):
return self.name
def save(self, *args, **kwargs):
self.name = self.name.upper()
self.addr = self.addr.upper()
self.city = self.city.upper()
self.state = self.state.upper()
return super(Customer, self).save(*args, **kwargs)
In the view, after defining customer as
customer = current_vehicle.customer_set.all()
I tried the following:
if 'customer' in request.POST:
if customer:
customer_form = CustomerForm(request.POST, instance=customer[0])
if customer_form.is_valid():
customer_form.save()
Also tried adding before customer_form is defined:
customer.vehicle_id = current_vehicle.id
And then this after the form:
customer_form.vehicle_id = current_vehicle.id
Form is not valid so it's not saved. Upon checking {{ form.errors}}, it always reports vehicle_id is required.
Finally, after the answer in this, I adjusted it to my scenario by adding:
obj = customer_form.save(commit=False)
and hoping to assign vehicle_id, but it fails immediately.
What am I missing?
Thanks.
1st EDIT:
The section on the view now looks as:
customer_form = CustomerForm(request.POST, instance=customer[0])
customer_form.save()
customer_form.vehicle_id.add(current_vehicle)

Answer 1

You are misunderstanding what a ManyToMany field is here:
customer_form.vehicle_id = current_vehicle.id
vehicle_id is defined as a ManyToMany field on your Customer model, therefore you can't just assign a single id to it. You have to add an instance of VehicleSale model, eg:
customer_form.vehicle_id.add(current_vehicle)
See docs here:
https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_many/
See also this answer for why you can't save until you populate the vehicle_id relation:
https://stackoverflow.com/a/2529875/202168