Explicitly use SECTION_ITEM_MODEL to ensure the desired section field…

…s are present on potential subclasses

armstrong/core/arm_sections/backends.py

@@ -1,6 +1,6 @@
 from django.db.models import Q
 
-from .utils import get_section_relations
+from .utils import get_section_relations, get_item_model_class
 
 
 class ItemFilter(object):
@@ -20,7 +20,7 @@ def filter_objects_by_section(self, rels, section):
         q = Q(**kwargs_list[0])
         for kwargs in kwargs_list[1:]:
             q |= Q(**kwargs)
-        return self.get_manager(rels[0].model).filter(q)
+        return self.get_manager(get_item_model_class()).filter(q)
 
     def process_items(self, items):
         """