Django save() always create a new object - django

Question

I have this model in my app which is meant to auto-generate it's primary Key based on a method added in the save().
However, for each object, I will be expected to make updates of certain fields. Right now, anytime I make an update on the admin side (testing use cases) it instead creates a new record of the PK instead of updating the existing one. Any thoughts on how to remedy this?
class DeploymentTask(models.Model):
deployment_id = models.CharField(
'Deployment Task ID', primary_key=True, max_length=25, editable=False)
title = models.CharField(max_length=100)
current_status = FSMField('Current Status',
default=STATES[0], choices=STATES)
site_id = models.ForeignKey(
Site, related_name='+', on_delete=models.CASCADE)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
refuel_record = models.ManyToManyField(RefuelRecord)
def __str__(self):
"""String for representing the Model object."""
return self.deployment_id
class Meta:
db_table = 'rm_deployment_task'
verbose_name_plural = 'Deployment Tasks'
def get_absolute_url(self):
return reverse('deployment_id-view', args=[str(self.deployment_id)])
def save(self):
today = datetime.datetime.now()
ticket_count = DeploymentTask.objects.filter(
created_at__year=today.year, created_at__month=today.month).count() + 1
new_task_id = 'DPT-' + str(str(datetime.date.today().year)) + str(
datetime.date.today().month).zfill(2) + str(
datetime.date.today().day).zfill(2) + '-' + str(ticket_count).zfill(6)
self.deployment_id = new_task_id
super(DeploymentTask, self).save()
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Answer 1

You are always setting self.deployment_id to a new value in the save method.
Django tries to do UPDATE ... WHERE deployment_id = %, but there are no records with this id yet (at least if you are saving "old" object on different date or having different ticket_count)
If you want to update fields other than deployment_id, then simply do not set self.deployment_id if it's already set. If you want to update deployment_id then there is not straightforward way to do this, because it's used as primary key (but you can remember old pk and delete that object after you have created a new one during save)
Read more in the Django docs.

Answer 2

This is my updated code, which worked for me...
def make_id():
today = datetime.datetime.now()
ticket_count = DeploymentTask.objects.filter(
created_at__year=today.year, created_at__month=today.month).count() + 1
new_task_id = 'DPT-' + str(str(datetime.date.today().year)) + str(
datetime.date.today().month).zfill(2) + str(
datetime.date.today().day).zfill(2) + '-' + str(ticket_count).zfill(6)
return new_task_id
class DeploymentTask(models.Model):
deployment_id = models.CharField(
'Deployment Task ID', primary_key=True, max_length=25, editable=False, default=make_id)
title = models.CharField(max_length=100)
current_status = FSMField('Current Status',
default=STATES[0], choices=STATES)
site_id = models.ForeignKey(
Site, related_name='+', on_delete=models.CASCADE)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
refuel_record = models.ManyToManyField(RefuelRecord)
def __str__(self):
"""String for representing the Model object."""
return self.deployment_id
class Meta:
db_table = 'rm_deployment_task'
verbose_name_plural = 'Deployment Tasks'
def get_absolute_url(self):
return reverse('deployment_id-view', args=[str(self.deployment_id)])