Apply duck-typing for model-like classes within ChooserViewset

* allow any class to pass through resolve_model_string, not just models
* use BlanketPermissionPolicy if model_class is not a real model

wagtail/admin/views/generic/chooser.py

@@ -9,6 +9,7 @@ from django.core.exceptions import (
     PermissionDenied,
 )
 from django.core.paginator import InvalidPage, Paginator
+from django.db.models import Model
 from django.forms.models import modelform_factory
 from django.http import Http404
 from django.template.loader import render_to_string
@@ -296,7 +297,7 @@ class CreationFormMixin(ModelLookupMixin, PreserveURLParametersMixin):
     def get_permission_policy(self):
         if self.permission_policy:
             return self.permission_policy
-        elif self.model_class:
+        elif issubclass(self.model_class, Model):
             return ModelPermissionPolicy(self.model_class)
         else:
             return BlanketPermissionPolicy(None)

wagtail/coreutils.py

@@ -82,7 +82,7 @@ def resolve_model_string(model_string, default_app=None):
 
         return apps.get_model(app_label, model_name)
 
-    elif isinstance(model_string, type) and issubclass(model_string, Model):
+    elif isinstance(model_string, type):
         return model_string
 
     else:

wagtail/tests/tests.py

@@ -520,7 +520,8 @@ class TestResolveModelString(TestCase):
         self.assertRaises(ValueError, resolve_model_string, "Page")
 
     def test_resolve_from_class_that_isnt_a_model(self):
-        self.assertRaises(ValueError, resolve_model_string, object)
+        model = resolve_model_string(object)
+        self.assertEqual(model, object)
 
     def test_resolve_from_bad_type(self):
         self.assertRaises(ValueError, resolve_model_string, resolve_model_string)