Django ModelForm inheritance, subclassing and passing in an instance

Question

I have an address model:

class Address(models.Model):
  line1 = models.CharField(max_length=100)
  ....

and an AddressForm:

class AddressForm(forms.ModelForm):
  class Meta:
    model = Address

Several models will have an address field, so instead of copy+pasting all of the fields from the AddressForm, I wanted to extend the AddressForm so I can reuse it. Specifically, I want to avoid copying+pasting all of the form fields into other app's forms and have only one AddressForm.is_valid() function.

For example, an Order will have an Address, so my plan was to do something like this:

class Order:
  address = models.ForeignKey(Address, on_delete=models.CASCADE)

class OrderForm(AddressForm):
  class Meta:
    model = Order

However, when I pass an instance to the OrderForm from the order/views.py, I'm passing instance of an Order and all of the AddressForm fields show up as blank:

form = OrderForm(instance=order)

How do I initialize the AddressForm fields from the order.address field?

I had tried this:

class OrderForm(AddressForm):
    def __init__(self, *args, **kwargs):
        kwargs['instance'] = kwargs['instance'].address
        super(OrderForm, self).__init__(*args, **kwargs)

But it doesn't work.

Is there a way to do this? Or a better way of reusing the AddressForm? Or would this only work if Order was a subclass of Address?

Cheers,

Alasdair · Accepted Answer · 2017-07-20 12:29:58Z

5

If you don't want to subclass Address, then you should use two forms in your view instead of trying to include the address fields in the order form.

if request.method = "POST":
    address_form = AddressForm(request.POST, instance=order.address)
    order_form = OrderForm(request.POST, instance=order)
    if address_form.is_valid() and order_form.is_valid():
        address = address_form.save()
        order = order_form.save()

Your order form should exclude the foreign key to Address.

class OrderForm(AddressForm):
    class Meta:
    model = Order
    exclude = ['address']

When you create new instances, then you should save the order form with commit=False, set the address, and then save the order:

if request.method = "POST":
    address_form = AddressForm(request.POST)
    order_form = OrderForm(request.POST)
    if address_form.is_valid() and order_form.is_valid():
        address = address_form.save()
        order = order_form.save(commit=False)
        order.address = address
        order.save()

answered Jul 20, 2017 at 12:29

Alasdair

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1 Comment

Travis D Over a year ago

Awesome, that's a great solution. Cheers!

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