Overriding ModelForm.save does not update all affected fields

Question

I want to store money amounts as integers in the database. For convenience I added float fields as well:

# File: models.py 
class Transaction(models.Model):
    user = models.ForeignKey(User, related_name='transactions')
    date = models.DateTimeField(default=datetime.now)
    # transacted btc-amount in satoshi; positive when I bought btc, negative else
    amount_btc_satoshi = models.IntegerField()
    # for convenience: transacted btc-amout in units of 1 btc
    amount_btc = models.FloatField(null=True)
    # transacted fiat-amount in 1e-5 euros; positive when I sold btc, negative else
    amount_eur_milicent = models.IntegerField()
    # for convenience: transacted fiat-amout in units of 1 eur
    amount_eur = models.FloatField(null=True)
    # True if I bought bitcoins, False if I sold bitcoins
    is_bid = models.BooleanField()
    # effective fiat price per 1 BTC in EUR
    price_per_btc = models.FloatField()

For convenience I overrode the save method in my ModelForm derivate. It is supposed to automatically update some dependent fields based on amount_btc and amount_eur:

# File: forms.py
class TransactionForm(forms.ModelForm):
    def clean(self):
        cleaned_data = super(TransactionForm, self).clean()
        if cleaned_data['amount_btc'] > 0. and cleaned_data['amount_eur'] > 0.:
            raise forms.ValidationError('Either BTC amount or fiat amount must be negative.')
        return cleaned_data

    def save(self, commit=True, *args, **kwargs):
        instance = super(TransactionForm, self).save(commit=False, *args, **kwargs)
        # store input data in integer format
        instance.amount_btc_satoshi = int(round(self.cleaned_data['amount_btc'] * 1e8))
        instance.amount_eur_milicent = int(round(self.cleaned_data['amount_eur'] * 1e5))
        # provide convenient amounts
        instance.amount_btc = instance.amount_btc_satoshi / 1e8
        instance.amount_eur = instance.amount_eur_milicent / 1e5
        instance.is_bid = instance.amount_btc_satoshi > 0
        instance.price_per_btc = -1. * instance.amount_eur / instance.amount_btc
        if commit:
            instance.save()
        return instance

    class Meta:
        model = Transaction
        fields = ['date', 'amount_btc', 'amount_eur']

Now adding a new transaction works as expected with is_bid and the other dependend fields properly set. However, editing an existing entry results in the update of only one field. E.g. neither price_per_btc nor amount_eur_milicent is changed even if amount_eur is (see below):

# File: views.py
@login_required
def transaction_add(request):
    form = TransactionForm(request.POST)
    if form.is_valid():
        transaction = form.save(commit=False)
        transaction.user = request.user
        transaction.save()
    else:
        messages.error(request, ';'.join('{}: {}'.format(key, value) for key, value in form.errors.items()))
    return redirect(request.POST['next'])

@login_required
def transaction_edit(request, id):
    transaction = Transaction.objects.get(id=id)
    form = TransactionForm(request.POST, instance=transaction)
    if form.is_valid():
        transaction.save()
    else:
        messages.error(request, ';'.join('{}: {}'.format(key, value) for key, value in form.errors.items()))
    return redirect(request.POST['next'])

I tried instance.save(update_fields=None), but it did not have any effect at all. Ideas?

Answer 1

The transaction instance won't get modified this way, You should call form.save() explicitly (cause the form actually contains the changed data)

transaction = Transaction.objects.get(id=id)
form = TransactionForm(request.POST, instance=transaction)
if form.is_valid():
    form.save()

You won't create a new object, just edit the existing one, since the objects primary key is not changed