Get tests running on django 1.11

I'm a little concerned that this loses some of the efficiencies (in
particular, chunking) from previous versions. That's something that can
probably be improved.

polymorphic/query.py

@@ -25,6 +25,25 @@ except ImportError:
 Polymorphic_QuerySet_objects_per_request = CHUNK_SIZE
 
 
+if django.VERSION >= (1, 9):
+    # We ignore this on django < 1.9, as ModelIterable didn't yet exist.
+    from django.db.models.query import ModelIterable
+
+
+    class PolymorphicModelIterable(ModelIterable):
+        def __iter__(self):
+            base_iter = super(PolymorphicModelIterable, self).__iter__()
+
+            if self.queryset.polymorphic_disabled:
+                for o in base_iter:
+                    yield o
+                return
+
+            real_instances = self.queryset._get_real_instances(base_iter)
+            for obj in real_instances:
+                yield obj
+
+
 def transmogrify(cls, obj):
     """
     Upcast a class to a different type without asking questions.
@@ -72,6 +91,9 @@ class PolymorphicQuerySet(QuerySet):
         # to that queryset as well).
         self.polymorphic_deferred_loading = (set([]), True)
         super(PolymorphicQuerySet, self).__init__(*args, **kwargs)
+        if django.VERSION >= (1, 9):
+            # On django < 1.9 we override the iterator() method instead
+            self._iterable_class = PolymorphicModelIterable
 
     def _clone(self, *args, **kwargs):
         # Django's _clone only copies its own variables, so we need to copy ours here
@@ -407,49 +429,51 @@ class PolymorphicQuerySet(QuerySet):
 
         return resultlist
 
-    def iterator(self):
+    if django.VERSION < (1, 9):
-        """
+        # On django 1.9+, we can define self._iterator_class instead of iterator()
-        This function is used by Django for all object retrieval.
+        def iterator(self):
-        By overriding it, we modify the objects that this queryset returns
+            """
-        when it is evaluated (or its get method or other object-returning methods are called).
+            This function is used by Django for all object retrieval.
+            By overriding it, we modify the objects that this queryset returns
+            when it is evaluated (or its get method or other object-returning methods are called).
 
-        Here we do the same as::
+            Here we do the same as::
 
-            base_result_objects = list(super(PolymorphicQuerySet, self).iterator())
+                base_result_objects = list(super(PolymorphicQuerySet, self).iterator())
-            real_results = self._get_real_instances(base_result_objects)
+                real_results = self._get_real_instances(base_result_objects)
-            for o in real_results: yield o
+                for o in real_results: yield o
 
-        but it requests the objects in chunks from the database,
+            but it requests the objects in chunks from the database,
-        with Polymorphic_QuerySet_objects_per_request per chunk
+            with Polymorphic_QuerySet_objects_per_request per chunk
-        """
+            """
-        base_iter = super(PolymorphicQuerySet, self).iterator()
+            base_iter = super(PolymorphicQuerySet, self).iterator()
 
-        # disabled => work just like a normal queryset
+            # disabled => work just like a normal queryset
-        if self.polymorphic_disabled:
+            if self.polymorphic_disabled:
-            for o in base_iter:
+                for o in base_iter:
-                yield o
+                    yield o
-            return
-
-        while True:
-            base_result_objects = []
-            reached_end = False
-
-            for i in range(Polymorphic_QuerySet_objects_per_request):
-                try:
-                    o = next(base_iter)
-                    base_result_objects.append(o)
-                except StopIteration:
-                    reached_end = True
-                    break
-
-            real_results = self._get_real_instances(base_result_objects)
-
-            for o in real_results:
-                yield o
-
-            if reached_end:
                 return
 
+            while True:
+                base_result_objects = []
+                reached_end = False
+
+                for i in range(Polymorphic_QuerySet_objects_per_request):
+                    try:
+                        o = next(base_iter)
+                        base_result_objects.append(o)
+                    except StopIteration:
+                        reached_end = True
+                        break
+
+                real_results = self._get_real_instances(base_result_objects)
+
+                for o in real_results:
+                    yield o
+
+                if reached_end:
+                    return
+
     def __repr__(self, *args, **kwargs):
         if self.model.polymorphic_query_multiline_output:
             result = [repr(o) for o in self.all()]