title: How to save() an argument in a Django Model
body: <p>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:</p>
  <p><code>models.py</code></p>
  <pre class="lang-py prettyprint-override"><code>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
  </code></pre>
  <p><code>views.py</code></p>
  <pre class="lang-py prettyprint-override"><code>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')
  </code></pre>
  <p>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.</p>