There's no `transaction_id` for unpaid preferences about django-payments-mercadopago HOT 1 OPEN

Hello, sorry for the late response, because of some reason I never noticed this issue notification neither the other one.

As you stated, I'm seeing that the self.create_payment is being created unconditionally. You're totally right, two preferences aren't needed.

I'm using this library along with a customized version of an old saleor version. I think the way saleor handled the payments allowed the tricky behavior to pass without causing any trouble. But let's change it so it can be used in any implementation of django-payments.

How about something like this?

def get_form(self, payment: BasePayment) -> None:
        if not payment.id:
            payment_data = self.create_payment(payment)
            redirect_to = self.get_value_from_response(
                payment_data, self.init_point)
            payment.change_status(PaymentStatus.WAITING)
            payment.save()
            raise RedirectNeeded(redirect_to)
        redirect_to = self.get_value_from_response(json.loads(payment.extra_data), self.init_point)
        raise RedirectNeeded(redirect_to)

If there is not payment.id (the payment has never been saved) we send the preference to mercadopago via create_payment, we get the the full response from mercadopago and save it in payment.extra_data, which contains the required payment url in "init_point" key. If there is a payment.id (a preference was previously created) we simply redirect the user to the same payment url we received from mercadopago response.

It appears to work, what do you think about it? I may be forget something.

from django-payments-mercadopago.