add select_related and partial prefetch_related support

implement support for a single query for select related base fetches across
polymorphic models.

adds a polymorphic QuerySet Mixin to enable non polymorphic models to fetch
related models.

Fixes: jazzband#198 jazzband#436 jazzband#359 jazzband#244
Possible Fixes:
  jazzband#498: support for prefetch_related cannot fetch attributes not on all child models or via class names
Related: jazzband#531

polymorphic/base.py

@@ -57,12 +57,6 @@ class PolymorphicModelBase(ModelBase):
     def __new__(self, model_name, bases, attrs, **kwargs):
         # print; print '###', model_name, '- bases:', bases
 
-        polymorphic__proxy = None
-        if "Meta" in attrs:
-            if hasattr(attrs["Meta"], "polymorphic__proxy"):
-                polymorphic__proxy = attrs["Meta"].polymorphic__proxy
-                del attrs["Meta"].polymorphic__proxy
-
         # Workaround compatibility issue with six.with_metaclass() and custom Django model metaclasses:
         if not attrs and model_name == "NewBase":
             return super().__new__(self, model_name, bases, attrs, **kwargs)
@@ -89,11 +83,6 @@ def __new__(self, model_name, bases, attrs, **kwargs):
         # for __init__ function of this class (monkeypatching inheritance accessors)
         new_class.polymorphic_super_sub_accessors_replaced = False
 
-        if polymorphic__proxy is not None:
-            new_class._meta.polymorphic__proxy = polymorphic__proxy
-        else:
-            new_class._meta.polymorphic__proxy = not new_class._meta.proxy
-
         # determine the name of the primary key field and store it into the class variable
         # polymorphic_primary_key_name (it is needed by query.py)
         for f in new_class._meta.fields:

polymorphic/managers.py

@@ -28,7 +28,7 @@ def from_queryset(cls, queryset_class, class_name=None):
 
     def get_queryset(self):
         qs = self.queryset_class(self.model, using=self._db, hints=self._hints)
-        if not self.model._meta.polymorphic__proxy:
+        if self.model._meta.proxy:
             qs = qs.instance_of(self.model)
         return qs
 

polymorphic/models.py

@@ -198,9 +198,18 @@ def __init__(self, *args, **kwargs):
         self.__class__.polymorphic_super_sub_accessors_replaced = True
 
         def create_accessor_function_for_model(model, accessor_name):
+            NOT_PROVIDED = object()
+
             def accessor_function(self):
-                objects = getattr(model, "_base_objects", model.objects)
-                attr = objects.get(pk=self.pk)
+                attr = NOT_PROVIDED
+                try:
+                    attr = self._state.fields_cache[accessor_name]
+                    pass
+                except KeyError:
+                    pass
+                if attr is NOT_PROVIDED:
+                    objects = getattr(model, "_base_objects", model.objects)
+                    attr = objects.get(pk=self.pk)
                 return attr
 
             return accessor_function