Question

Model:

class Subject(models.Model):
    name = models.CharField(max_length=50)
    places = models.IntegerField()

class Temp(models.Model):
    subject_r = models.ForeignKey(Subject)

Now in my views I want to do simple thing:
If Subject id exists in Temp table, field places (which is integer) need to be decremented by 1, else, it should stay as it is, and then I want to display the correct places variable in my template. Note that i don’t want to save new value in a database, i just want it to be displayed correctly in a template.

Answer 1

temp = Temp.objects.values_list('subject_r_id', flat=True)
subjects = Subject.objects.all()
for subject in subjects:
    if subject.id in temp:
        subject.places = subject.places - 1

Since it’s for display only, I would actually set an arbitrary new attribute so that it’s clear you’re not trying to modify the database.

temp = Temp.objects.values_list('subject_r_id', flat=True)
subjects = Subject.objects.all()
for subject in subjects:
    if subject.id in temp:
        subject.places_display = subject.places - 1
    else:
        subject.places_display = subject.places


{{ subject.places_display }}