How to write and retrieve data from ManyToManyField inside overrided save() method? - django

Question

I need to write any data to ManyToManyField via Model's form in the template, but i get an error like "... needs to have a value for field "id" before this many-to-many relationship can be used.". It shows when I try to use self.service("service" is my ManyToManyField) in my overrided save() method. I know that ManyToManyField is not basic field and it returns something like queryset, but how can i manipulate data inside save() method, because "self.service" doesn't work.
# models.py
class Appointments(models.Model):
name = models.CharField(max_length=200, db_index=True, verbose_name='Имя, фамилия')
tel = models.CharField(max_length=200, db_index=True, verbose_name='Номер телефона')
e_mail = models.CharField(max_length=200, blank=True, db_index=True, verbose_name='E-mail')
car = models.ForeignKey('Cars', null=True, on_delete=models.PROTECT, verbose_name='Тип автомобиля')
num_car = models.CharField(max_length=200, null=True, db_index=True, verbose_name='Гос.номер автомобиля')
**service = models.ManyToManyField(Services, verbose_name='Тип услуги')**
date_created = models.DateTimeField(auto_now_add=True, db_index=True, verbose_name='Дата публикации заявки')
date_service = models.DateField(db_index=True, verbose_name='Дата')
time_service = models.TimeField(db_index=True, help_text="Введите время в таком формате: 15:00", verbose_name='Время')
price = models.CharField(max_length=50, db_index=True, null=True, verbose_name='Цена')
def save(self, *args, **kwargs):
for i in Services_prices.objects.all():
ccar = i.car
sservice = i.service
for d in self.service:
if self.car == ccar and d == sservice:
self.price = i.price
break
elif ccar == None and d == sservice:
self.price = i.price
break
super().save(*args, **kwargs)
# forms.py
class AppointmentForm(forms.ModelForm):
service = forms.ModelMultipleChoiceField(queryset=Services.objects.all(), required=False, widget=forms.CheckboxSelectMultiple())
class Meta:
model = Appointments
fields = ('name', 'tel', 'e_mail', 'car', 'num_car', 'service', 'date_service', 'time_service')

Answer 1

In order to have a many_to_many relation between two objects, you need primary keys of the both objects. Before calling super's save, your model does not have a primary key yet.
In your overriden save method, call super first, (e.g.super().save(*args, **kwargs)) then do your stuff, then save again.