Avoid ContentType db query, use cache instead

Author: vdboor

polymorphic_tree/models.py

@@ -122,8 +122,7 @@ def get_child_types(self):
  except ValueError:
  app_label = self._meta.app_label
  model = child
- ct_id = ContentType.objects.get(app_label=app_label,
- model=model).id
+ ct_id = ContentType.objects.get_by_natural_key(app_label, model).id
  else:
  # pass in a model class
  ct_id = ContentType.objects.get_for_model(child).id