Django change choice before validation - django

Question

I am trying to update a choice field before it is validated in Django. The reason for this is the choice field in question value is the Id of a model that i want to update.
def fuelLogSystemOne(request):
entries = FuelLogSystemOneMod.objects.all().order_by('date')
products = productsMod.objects.all()
if request.method == 'POST':
form = forms.AddFuelLogOneForm(request.POST, request.FILES)
productid = form['product'].value()
product = productsMod.objects.get(id=productid)
product_id = request.POST.get('product')
form.fields['product'].choices = [(product.product_type, product.product_type)]
if form.is_valid():
bucketsRemoved = form['buckets_added'].value()
product.stock -= bucketsRemoved
product.save(['stock'])
instance = form.save(commit=False)
instance.staff = request.user
instance.save()
return redirect('home')
else:
form = forms.AddFuelLogOneForm()
return render(request,'systems/fuellogsystemone.html',{'form':form,'entry':entries,'products':products})
The below part is where i am trying to change the form data before it gets validated so it doesn't say 'Select a valid choice. 1 is not one of the available choices'
product_id = request.POST.get('product')
form.fields['product'].choices = [(product.product_type, product.product_type)]
But when I first submit the form it is still saying 'Select a valid choice.'
At what point does Django validate the form because I am changing the form before the is_valid() method and it still hits this error?

Answer 1

This should be in your form, you have to override the init method, and pass the product id during form initialization in the views
forms.py
class AddFuelLogOneForm(forms.ModelForm):
def __init__(self, *args, **kwargs):
product = kwargs.pop('product', None)
self.fields['product'].choices = [(product.product_type, product.product_type)]
views.py
def fuelLogSystemOne(request):
entries = FuelLogSystemOneMod.objects.all().order_by('date')
products = productsMod.objects.all()
if request.method == 'POST':
product_id = request.POST.get('product')
productid = form['product'].value()
product = productsMod.objects.get(id=productid)
product_id = request.POST.get('product')
form = forms.AddFuelLogOneForm(request.POST, request.FILES, product=product)
if form.is_valid():
bucketsRemoved = form['buckets_added'].value()
product.stock -= bucketsRemoved
product.save(['stock'])
instance = form.save(commit=False)
instance.staff = request.user
instance.save()
return redirect('home')
else:
form = forms.AddFuelLogOneForm()
return render(request,'systems/fuellogsystemone.html',{'form':form,'entry':entries,'products':products})