Django: How to update an Integerfield and decrement its value - django

Question

This seemingly innocuous problem has turned out to be quite difficult to find any information on. I just want to decrement the value of an Integerfield column by 1 in my database, by calling a function.
views.py function call
StudentProfile.objects.add_lesson(student_id)
managers.py
class StudentQuerySet(models.QuerySet):
def add_lesson(self, sid):
self.filter(student_id=sid).update(remaining_lessons=remaining - 1)
class StudentProfileManager(models.Manager):
def add_lesson(self, sid):
self.get_queryset().add_lesson(sid)
Full StudentProfile model
class StudentProfile(models.Model):
student = models.OneToOneField(
User, related_name='student', primary_key=True, parent_link=True, on_delete=models.CASCADE)
portrait = models.ImageField(
upload_to='studentphotos', verbose_name=_('Student Photo'))
about_me = models.TextField(verbose_name=_("About Me"))
spoken_languages = models.CharField(max_length=255)
teacher_default = models.OneToOneField(
'teachers.TeacherProfile', related_name='teacher_default', parent_link=True,
on_delete=models.CASCADE, default=None, blank=True, null=True)
membership_start = models.DateTimeField(
verbose_name="Membership Start Date", default=now, editable=False)
membership_end = models.DateTimeField(
verbose_name="Membership End Date", default=now, editable=False)
remaining_lessons = models.IntegerField(
verbose_name="Membership remaining lessons", default=0)
objects = StudentProfileManager()
def __str__(self):
return User.objects.get_student_name(self.student_id)
I know this is totally wrong, any help is appreciated.

Answer 1

If you want to keep your current setup and be able to add_lesson() to decrement "remaining_lessons", the smallest change you can do to achieve it is by using F() expression:
from django.db.models import F
class StudentQuerySet(models.QuerySet):
def add_lesson(self, sid):
self.filter(student_id=sid).update(remaining_lessons=F('remaining_lessons') - 1)
Ref: https://docs.djangoproject.com/en/3.0/ref/models/expressions/
Although I personally think that if your goal is only to have a method that decrement "remaining_lessons" by 1, you should probably just make it a model method. Like this:
class StudentProfile(models.Model):
# ... your model field ...
def add_lesson(self):
self.remaining_lesson -= 1
self.save()
# and in your Views.py
StudentProfile.objects.get(student_id=sid).add_lesson()
Hope this helps.

Answer 2

Django provides F expressions for exactly the kind of task you have. It makes the update relative to the original field value in the database.
You would need to change your managers.py as follows (plus the return statements :) )
from django.db.models import F
class StudentQuerySet(models.QuerySet):
def add_lesson(self, sid):
return self.filter(student_id=sid).update(remaining_lessons=F('remaining_lessons')-1)
class StudentProfileManager(models.Manager):
def add_lesson(self, sid):
return self.get_queryset().add_lesson(sid)
You could go even further, and for the sake of DRY approach, use QuerySet.as_manager() to create an instance of Manager with a copy of a custom QuerySet’s methods instead of repeating the method twice in your custom Manager and QuerySet. E.g.:
class StudentProfile(models.Model):
...
objects = StudentQuerySet().as_manager()
Hope it helps!

Answer 3

I tried using the F expression, and I have no clue why, but it was decrementing by 3 instead of by 1. Maybe Django runs that code 3 times when it is called in the view.
I found a solution that accomplishes this without a function, in the view, it does exactly what I expect, a decrement of 1:
student_id = request.user.id
student_queryset = StudentProfile.objects.get(student_id=student_id)
student_queryset.remaining_lessons = student_queryset.remaining_lessons - 1
student_queryset.save()