Problem with save method overriding attribute value - django

Question

I have a model called 'Trip' with a Foreign Key to 'Destination'. The Destination model specifies a maximum number of passengers in it's 'max_passengers' attribute.
Trip
class Trip(models.Model):
destination = models.ForeignKey(
Destination,
null=True,
blank=False,
on_delete=models.SET_NULL,
related_name="trips",
)
date = models.DateTimeField()
seats_available = models.IntegerField(
null=False, blank=False, editable=False
)
trip_ref = models.CharField(
max_length=32, null=True, editable=True, blank=True
)
Destination
class Destination(Product):
max_passengers = models.IntegerField(null=True, blank=True)
duration = models.CharField(max_length=20, blank=True)
addons = models.ManyToManyField(AddOn)
min_medical_threshold = models.IntegerField(
default=0, null=False, blank=False
)
def __str__(self):
return self.name
Back in the Trip model, I am overriding the model save method, so that when a trip object is created, the 'seats_available' for that instance is set to the 'max_passengers' of the related destination:
def save(self, *args, **kwargs):
if not self.trip_ref:
date = (self.date).strftime("%m%d-%y")
self.trip_ref = self.destination.pk + "-" + date
if not self.seats_available:
self.seats_available = self.destination.max_passengers
super().save(*args, **kwargs)
I have additional models for Bookings and Passengers. When a booking is created, a post_save signal is sent, calling the model method on my Trips model, def update_seats_avaialable():
def update_seats_available(self):
reservations = (
self.bookings.aggregate(num_passengers=Count("passengers"))
["num_passengers"]
)
self.seats_available = self.destination.max_passengers - reservations
self.save() <---- PROBLEM
THe problem is when all seats are finally taken ie. the passenger count = max_passengers and seats availabe = 0. When self.save triggers the save method, if not self.seats_available = true and so this line of code is run: self.seats_available = self.destination.max_passengers which sets the seats available back to where it started. Is there another way of initialising the model object? My understanding is init method isnt advised...

Answer 1

Okay I came up with a solution:
def update_seats_available(self):
reservations = (
self.bookings.aggregate(num_passengers=Count("passengers"))
["num_passengers"])
self.save(reservations=reservations)
def save(self, *args, **kwargs):
if not self.trip_ref:
date = (self.date).strftime("%m%d-%y")
self.trip_ref = self.destination.pk + "-" + date
reservations = kwargs.pop('reservations', 0)
self.seats_available = self.destination.max_passengers - reservations
super().save(*args, **kwargs)
Here I'm passing the value of reservation to the save method and doing the calculation there instead.