UnsupportedChildType when using formsets?

[kimutaiRop]

I have a polymorphic model that I made and it has many child models which I want the users to be choosing dynamically, posting the first product has no issue, I was able to select the form to send to the user, however when I try subsequent queries for the model to send to user I get error

polymorphic.formsets.models.UnsupportedChildType: The 'ShirtsAndBlouseForm' found a 'PhoneTablets' model in the queryset, but no form class is registered to display it.
my forms look like this

PhonesTabsForm = polymorphic_modelformset_factory(Product, fields=product_fields, formset_children=(
    PolymorphicFormSetChild(PhoneTablets, fields=product_fields + ['screen_size', 'battery_type',
                                                                   'battery_capacity', 'ram_measure', 'ram_storage',
                                                                   'internal_storage']),
))

ShirtsAndBlouseForm = polymorphic_modelformset_factory(Product, fields=product_fields, formset_children=(
    PolymorphicFormSetChild(ShirtsAndBlouse, fields=product_fields + ['techniques',
                                                                      'fit_type', 'length', 'material', 'fabric_type',
                                                                      'wear_type', 'design'], ),
))

I dont understand what could be happening that it can allow me to select the form during the first time but not the second time when product exist in the database

[lukatavcer]

I had the same issue as I forgot to include the child class (StackedPolymorphicInline.Child) in the inline (StackedPolymorphicInline).

[JohananOppongAmoateng]

@bckohan i think this is a user error not a bug in the library. I believe we can close this

[bckohan]

@kimutaiRop

That error is basically saying:

you instantiated ShirtsAndBlouseForm, but the queryset you gave it (or the default one it uses) contains at least one PhoneTablets object — and ShirtsAndBlouseForm only knows how to render ShirtsAndBlouse, so it blows up.

Why it “worked the first time”
On your first request there were no PhoneTablets rows yet (or no rows of other child types), so the default queryset happened to contain only the type your formset knows about. Once you saved a PhoneTablets, the next time you create ShirtsAndBlouseForm with the default queryset, it now includes that PhoneTablets row → UnsupportedChildType. This is expected behavior for django-polymorphic formsets.

The fix: always filter the queryset to the child type for that formset

Even though you create the factory like this, the filtering is not “baked in”:

ShirtsAndBlouseForm = polymorphic_modelformset_factory(
    Product,
    fields=product_fields,
    formset_children=(
        PolymorphicFormSetChild(ShirtsAndBlouse, fields=product_fields + [...]),
    )
)

You must do this when instantiating the formset:

# GET
formset = ShirtsAndBlouseForm(
    queryset=Product.objects.instance_of(ShirtsAndBlouse)
)

# POST (must use the same queryset concept!)
formset = ShirtsAndBlouseForm(
    request.POST,
    request.FILES,
    queryset=Product.objects.instance_of(ShirtsAndBlouse)
)

And similarly:

formset = PhonesTabsForm(
    queryset=Product.objects.instance_of(PhoneTablets)
)

instance_of() is the standard django-polymorphic queryset filter for child types.

If you’re using the formset only to create new items

Use an empty queryset so it never tries to render existing objects of other types:

formset = ShirtsAndBlouseForm(queryset=Product.objects.none())

Alternative: use one formset that registers all child types

If your UI is “one page that can edit mixed products”, then you should register all children in a single formset, and only display the subset you want in the template (or via initial client-side filtering). The library’s rule is: every model instance in the queryset must have a registered child form.

---

Here's a checklist

• Separate formset per product category? Filter queryset per category (instance_of(...)).
• Create-only flow? Use queryset=Product.objects.none().
• Mixed edit list? Use a single polymorphic formset registering all child types.