Django: How to verify a form whose fields are generated from a queryset in the forms __init__ method?

Question

I have some trouble coming up with a good solution to validating a form whose fields are set in its init method.

---

Quickly explaining my models: Every Product is supposed to have Variations, with each variation in turn having multiple VariationOptions.

• Every Product can have multiple Variations, like 'Color' or 'Material'
• Variations can have multiple VariationOptions (ManyToMany-Relation), like 'red' or 'concrete'

When rendering the AddToCartForm, every Variation corresponds to a ChoiceField with the VariationOptions being used as choices for said ChoiceField. (see forms.py)

---

Since every product can have different variations and therefore different fields, the AddToCart Form must be generated dynamically from a queryset containing the products variations (views.py), or at least thats what I'm thinking right now.

My Questions:

• How can I validate this kind of form?
• Do you see a better way of achieving the same idea?

I would very much appreciate your help. Thanks in advance!

---

Here is my simplified code:

MODELS.PY

class Product(models.Model):
    ..

class Variation(models.Model):
    name     =  models.CharField()
    product  =  models.ForeignKey(Product, on_delete=models.CASCADE, related_name='variations')
    options  =  models.ManyToManyField(VariationOption, blank=True)

class VariationOption(models.Model):
    value = CharField()

FORMS.PY

class AddToCart(forms.Form):
    quantity    = forms.PositiveIntegerField()

    # for every variation get all its choices and add new ChoiceField to form
    def __init__(self, variations, *args, **kwargs):
        super(AddToCart, self).__init__(*args, **kwargs)
        for variation in variations: 
            CHOICES = []
            choices_list = list(variation.options.all())
            for choice in choices_list:
                CHOICES.append(tuple((choice.value, choice.value)))
            self.fields[variation.name] = forms.ChoiceField(choices=CHOICES)

VIEWS.PY

def render_product(request):
    ...
    variations = list(Variation.objects.prefetch_related('options').filter(product=product))
    add_to_cart_form = AddToCartForm(variations=variations)
    return render(...)

@require_POST
def add_to_cart_view(request):
    form = AddToCartForm(request.POST) # This isn't working. I see why, but know no other way.
    if form.is_valid():
        ....

Answer 1

UPDATE

In case anyone comes across this in the future and has the same question: Apparently you can instantiate a form and pass it POST data in the same step.

With a form like this:

FORMS.PY

class SomeForm(forms.Form):
    field    = forms.PositiveIntegerField() #optional

    def __init__(self, variable, *args, **kwargs):
        super(AddToCart, self).__init__(*args, **kwargs)
        # do something with variable, generate fields or choices etc etc

This will work:

form = SomeForm(variable=something, data=request.POST)
if form.is_valid():
    # This will work when POST is actually valid

The form will pass the variable to its __init__method, does whatever logic is implemented and is then bound to the POST data. Validation now works as you would expect.