How to update a field in the model using ForegnKey lookup? - django

Question

I need to be able to update a field num_places in the Event model accessing it via event ForeignKey in the Participant model when the last is saved.
Here are my models.py:
class Event(models.Model):
name = models.CharField(max_length=50)
description = models.CharField(max_length=500)
text = models.TextField()
image = models.ImageField(blank=True)
date = models.DateTimeField()
price = models.PositiveIntegerField()
num_places = models.PositiveIntegerField(default=50)
slug = models.SlugField()
class Participant(models.Model):
name = models.CharField(max_length=200)
participant_uuid = models.UUIDField(primary_key=False, verbose_name='UUID')
email = models.EmailField()
phone_regex = RegexValidator(regex=r'^\+7\d{10}$')
phone_number = models.CharField(validators=[phone_regex], max_length=12)
num_places = models.PositiveIntegerField(default=1)
event = models.ForeignKey(Event, on_delete=models.CASCADE)
paid = models.BooleanField(default=False)
def __str__(self):
return self.name
def save(self, *args, **kwargs):
try:
self.full_clean(exclude=None)
self.event.num_places -= self.num_places # the value isn't updated
super().save(*args, **kwargs)
self.valid = True
self.non_field_errors = False
except ValidationError as e:
self.non_field_errors = e.message_dict[NON_FIELD_ERRORS]
self.valid = False
class Meta:
unique_together = ('name', 'email', 'phone_number', 'event')
The code with a comment has a problem: the num_places value in the Event model stays unchanged. How to fix it?

Answer 1

you need to save the modified object of Event.
self.event.save()
def save(self, *args, **kwargs):
try:
self.full_clean(exclude=None)
self.event.num_places -= self.num_places # the value isn't updated
self.event.save()
"""watch out this since you wanna do it at the end after setting valid, and non_fieild_errors"""
super().save(*args, **kwargs)
self.valid = True
self.non_field_errors = False
except ValidationError as e:
self.non_field_errors = e.message_dict[NON_FIELD_ERRORS]
self.valid = False