How to save() an argument in a Django Model

Question

I am learning Django, and I decided to simulate an ATM. When I run, my code works, and it says that the deposit to the Card was successful, but it does not save it, the original balance stays the same. Here's what I have:

models.py

class Card(models.Model):
    card_number = models.PositiveIntegerField(primary_key=True) 
    #account_number = models.ForeignKey(Account, to_field='account_number',on_delete=models.CASCADE)
    card_name = models.CharField(max_length=30)
    balance = models.PositiveIntegerField() 

    def __str__(self):
        return self.card_name

views.py

def deposit(request):
    if request.method == 'POST':
        user_pin = request.POST.get('Pin')
        user_card_number = request.POST.get('Your Card Number') 
        amount = request.POST.get('Amount')    
            user_balance = Card.objects.get(card_number=user_card_number).balance  
            user_card_name = Card.objects.get(card_number=user_card_number).card_name 

            # performs deposit
            user_balance += int(amount)                
            Card.objects.get(card_number=user_card_number).save()

            messages.success(request, 'Deposit Success! ' + '$'+ amount + ' has been depositted from card: ' + user_card_name)   
        else:
            messages.error(request, 'Card Number or Pin is incorrect')
    return render(request, 'deposit.html')

Now, I tried reading the save() documentation and follow tutorials, but they did not work. So, how can I save the new balance on my card after the deposit is done? Thank you.

Answer 1

you dont need get the Card object everytime you want to access an attribute of the model class.

You can do something like this

card= Card.objects.get(card_number=user_card_number)
card.balance +=int(amount)
card.save()

Answer 2

When you are want to save an update. First thing to do is get the object to be edit.

#gets user pin,balance, card name based on user's card number input
#pin_number = Card.objects.filter(card_number=user_card_number).pin  # error if use this
user_balance = Card.objects.get(card_number=user_card_number).balance  
user_card_name = Card.objects.get(card_number=user_card_number).card_name

You should change that code to

userCardObject = Card.objects.get(card_number=user_card_number)

then you can update the value inside the objects, and save it

userCardObject.balance += int(amount)
userCardObject.save()

What you do on your code just updating value of user_balance but doesn't update the value of row in table(because you don't update the column of balance directly to the object representation of it)