Model classes for an expense tracking app in Django

Question

I want to build a simple expense tracking app in Django. Each user can create categories, and each category can have multiple expenses:

enter image description here

Additionally, I wanted to have a value field that shows the total value of the category per month. For this I am using the def save() function in the Expenses class and a def calc_category_value() function called from the signals. I was wondering if this approach is correct:

def calc_category_value(instance):
 today = date.today()
 days = calendar.monthrange(today.year,today.month)[1]
 range_start = "%s-%s-%s" % (today.year,today.month,1)
 range_end = "%s-%s-%s" % (today.year,today.month,days)
 category = Category.objects.get(id=instance.category.id)
 expenses = Expense.objects.filter(date__range=[range_start,range_end],category=category)
 total = 0
 for e in expenses:
 total += e.amount 
 category.value = total
 category.save()
class Profile(models.Model):
 id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 user = models.OneToOneField(User, on_delete=models.CASCADE)
 create_date = models.DateTimeField('date added', auto_now_add=True)
 modify_date = models.DateTimeField('date modified', default=timezone.now)
class Category(models.Model):
 id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 user = models.ForeignKey(Profile, on_delete=models.CASCADE)
 name = models.CharField(max_length=200, unique=True)
 value = models.DecimalField(max_digits=11,decimal_places=2, default=0)
 create_date = models.DateTimeField('date added', auto_now_add=True)
 modify_date = models.DateTimeField('date modified', default=timezone.now)
 class Meta:
 verbose_name_plural = "Categories"
 def __unicode__(self):
 return self.name +' (' + str(self.value) + ')'
class Expense(models.Model):
 id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 title = models.CharField(max_length=200, blank=True)
 amount = models.DecimalField(max_digits=11, decimal_places=2,default=0)
 category = models.ForeignKey(Category, on_delete=models.CASCADE)
 date = models.DateField(default=timezone.now)
 create_date = models.DateTimeField('date added', auto_now_add=True)
 modify_date = models.DateTimeField('date modified', default=timezone.now)
 def save(self, *args, **kwargs):
 super(Expense, self).save(*args, **kwargs)
 today = date.today()
 days = calendar.monthrange(today.year,today.month)[1]
 range_start = "%s-%s-%s" % (today.year,today.month,1)
 range_end = "%s-%s-%s" % (today.year,today.month,days)
 expenses = Expense.objects.filter(date__range=[range_start,range_end],category=self.category)
 total = 0
 for e in expenses:
 total += e.amount
 self.category.value = total
 self.category.save()
 class Meta:
 verbose_name_plural = "Expenses"
 order_with_respect_to = 'category'
 def __unicode__(self):
 return self.title +', ' + str(self.amount) +' (' + self.category.name + ')'

Signals.py:

@receiver(post_delete, sender=Expense)
def post_delete(sender, instance, **kwargs):
 calc_category_value(instance)

Answer 1

You would probably want to show total expenses in the category for each month, not just the last. So Category doesn't have a value. It has a value (which is a bad naming by the way) for each month. This can be modelled simply as a query like (pseudo sql) select sum(amount) from expenses where group = 'some group' group by date.month

And by the way you can use this declarative style here.

total = 0
for e in expenses:
 total += e.amount 
category.value = total
category.value = sum(e.amount for e in expenses)