Django model validate a ManyToMany field before adding - django

Question

I have a model that looks somewhat like this:
class Passenger(models.Model):
name = models.CharField(max_length=50)
surname = models.CharField(max_length=50)
class Flight(models.Model):
capacity = models.IntegerField()
passengers = models.ManyToManyField(Passenger)
Before adding a new passenger to the flight I would like to validate whether the number of passengers is not going to exceed the capacity. I was wondering what would be the best way to go about this.
Obviously I could manually check the number of passengers before adding a new one, but maybe there is some support in django? I tried writing a validator, but wasn't sure how to do it.

Answer 1

According to Django docs you can listen to the m2m_changed signal, which will trigger pre_add and post_add actions.
Using add() with a many-to-many relationship, however, will not call
any save() methods (the bulk argument doesn’t exist), but rather
create the relationships using QuerySet.bulk_create(). If you need to
execute some custom logic when a relationship is created, listen to
the m2m_changed signal, which will trigger pre_add and post_add
actions.

Answer 2

According to #M.Void answer – Code Example:
from django.db import models
from django.db.models.signals import m2m_changed
from django.core.exceptions import ValidationError
class MyModel(models.Model):
m2mField = models.ManyToManyField('self')
m2mFieldLimit = 2
def m2mField_changed(sender,**kwargs):
instance = kwargs['instance']
if len(instance.m2mField.all()) >= instance.m2mFieldLimit :
raise ValidationError(f'Max number of records is {instance.m2mFieldLimi}')
m2m_changed.connect(commonobjects_changed,sender=MyModel.m2mField.through)

Answer 3

Override the clean method on the model to do the check you want:
class Passenger(models.Model):
name = models.CharField(max_length=50)
surname = models.CharField(max_length=50)
def clean(self, *args, **kwargs):
# clean gets called automatically by other things, so we can't always
# expect flight_id to be provided
if 'flight_id' in kwargs:
flight = Flight.objects.get(pk=kwargs['flight_id'])
if flight.passengers.all().count() >= flight.capacity:
# flight is full!
raise ValidationError
super(Passenger, self).clean()
class Flight(models.Model):
capacity = models.IntegerField()
passengers = models.ManyToManyField(Passenger)
Note that to do this, you will have to pass in the flight ID when validating the passenger:
f = Flight.objects.get(...)
p = Passenger(name='First', surname='Last')
try:
p.clean(flight_id=f.id) # full_clean calls clean, among other validations
p.save()
except ValidationError as e:
# do something to handle the error
Note that it is possible in multi-threaded applications for something to get validated successfully, but still fail to save in a race condition. You would need to add additional code to handle that.
See here for details on model validation.