Add UnsupportedChildType error for formsets

This also replaces the PolymorphicInlineModelAdmin.get_get_child_inline_instance()
and BasePolymorphicModelFormSet._construct_form() lookup with UnsupportedChildType

polymorphic/formsets/__init__.py

@@ -12,6 +12,7 @@ from .models import (
     BasePolymorphicModelFormSet,
     BasePolymorphicInlineFormSet,
     PolymorphicFormSetChild,
+    UnsupportedChildType,
     polymorphic_modelformset_factory,
     polymorphic_inlineformset_factory,
     polymorphic_child_forms_factory,
@@ -27,6 +28,7 @@ __all__ = (
     'BasePolymorphicModelFormSet',
     'BasePolymorphicInlineFormSet',
     'PolymorphicFormSetChild',
+    'UnsupportedChildType',
     'polymorphic_modelformset_factory',
     'polymorphic_inlineformset_factory',
     'polymorphic_child_forms_factory',

polymorphic/formsets/models.py

@@ -5,9 +5,16 @@ from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ImproperlyConfigured, ValidationError
 from django.forms.models import ModelForm, BaseModelFormSet, BaseInlineFormSet, modelform_factory, modelformset_factory, inlineformset_factory
 from django.utils.functional import cached_property
+
+from polymorphic.models import PolymorphicModel
 from .utils import add_media
 
 
+class UnsupportedChildType(LookupError):
+    pass
+
+
+
 class PolymorphicFormSetChild(object):
     """
     Metadata to define the inline of a polymorphic child.
@@ -167,7 +174,7 @@ class BasePolymorphicModelFormSet(BaseModelFormSet):
                 model = ContentType.objects.get_for_id(ct_id).model_class()
                 if model not in self.child_forms:
                     # Perform basic validation, as we skip the ChoiceField here.
-                    raise ValidationError("Child model type {0} is not part of the formset".format(model))
+                    raise UnsupportedChildType("Child model type {0} is not part of the formset".format(model))
         else:
             if 'instance' in defaults:
                 model = defaults['instance'].get_real_concrete_instance_class()  # respect proxy models
@@ -200,7 +207,18 @@ class BasePolymorphicModelFormSet(BaseModelFormSet):
         """
         if not self.child_forms:
             raise ImproperlyConfigured("No 'child_forms' defined in {0}".format(self.__class__.__name__))
-        return self.child_forms[model]
+        if not issubclass(model, PolymorphicModel):
+            raise TypeError("Expect polymorphic model type, not {0}".format(model))
+
+        try:
+            return self.child_forms[model]
+        except KeyError:
+            # This may happen when the query returns objects of a type that was not handled by the formset.
+            raise UnsupportedChildType(
+                "The '{0}' found a '{1}' model in the queryset, "
+                "but no form class is registered to display it.".format(
+                    self.__class__.__name__, model.__name__
+                ))
 
     def is_multipart(self):
         """