Negative Stock Prevention

Hello,

Please help crack the below code, I want to prevent negative stock, and if the stock is == 0, deduct it from reorder_level instead.

Currently, the stock goes negative.

models.py

class Stock(models.Model):
    quantity = models.IntegerField(default='0', blank=True, null=True)
    reorder_level = models.IntegerField(default='0', blank=True, null=True)

class Dispense(models.Model):
    drug_id = models.ForeignKey(Stock, on_delete=models.SET_NULL,null=True,blank=False)
    dispense_quantity = models.PositiveIntegerField(default='1', blank=False, null=True)
    taken=models.CharField(max_length=300,null=True, blank=True)

views.py

try:  
    
    if request.method == 'POST':
        if form.is_valid(): 
            username = form.cleaned_data['taken']
            qu=form.cleaned_data['dispense_quantity']
            ka=form.cleaned_data['drug_id']
            # print(username)
        
        
                
            stock= eo=Stock.objects.annotate(
            expired=ExpressionWrapper(Q(valid_to__lt=Now()), output_field=BooleanField())
            ).filter(expired=False).get(id=username)
            form=DispenseForm(request.POST or None, instance=stock)
            instance=form.save()
            # print(instance)
            if quantity > 0
                instance.quantity-=qu
                instance.save()
            else:
                instance.reorder_level-=qu
                instanc.save()

            form=DispenseForm(request.POST or None ,initial={'patient_id':queryset})
            form.save()

            messages.success(request, "Drug Has been Successfully Dispensed")

            return redirect('manage_patient_pharmacist')
        else:
            messages.error(request, "Validity Error")

            return redirect('manage_patient_pharmacist')

    context={
        "patients":queryset,
        "form":form,
        # "stocks":stock,
        "drugs":drugs,
        "prescrips":prescrips,
        "expired":ex,
        "expa":eo,

        }
    if request.method == 'POST':
    
        print(drugs)
        context={
            "drugs":drugs,
            form:form,
            "prescrips":prescrips,
            "patients":queryset,
            "expired":ex,
            "expa":eo,

        }
except:
    messages.error(request, "Dispensing Not Allowed! The Drug is Expired ,please do a re-stock ")
    return redirect('manage_patient_pharmacist')
context={
        "patients":queryset,
        "form":form,
        # "stocks":stock,
        "drugs":drugs,
        "prescrips":prescrips,
        "expired":ex,
        "expa":eo,

        }

return render(request,'templates/discharge.html',context)

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