Fix for proxy model defined in external app.

crudbuilder/abstract.py

@@ -1,3 +1,9 @@
+import django
+try:
+    from django.apps import apps
+except ImportError:
+    pass
+
 import crudbuilder
 from six import with_metaclass
 from django.contrib.contenttypes.models import ContentType
@@ -38,8 +44,15 @@ def __init__(
     @property
     def get_model_class(self):
         """Returns model class"""
-        c = ContentType.objects.get(app_label=self.app, model=self.model)
-        return c.model_class()
+        try:
+            c = ContentType.objects.get(app_label=self.app, model=self.model)
+        except ContentType.DoesNotExist:
+            # try another kind of resolution
+            # fixes a situation where a proxy model is defined in some external app.
+            if django.VERSION >= (1, 7):
+                return apps.get_model(self.app, self.model)
+        else:
+            return c.model_class()
 
     def _has_crud_attr(self, attr):
         return getattr(self.crud, attr, None)