fix-#397: Allow specifying the '_cls' as a field for indexes

mongoengine/base/document.py

@@ -26,7 +26,7 @@ NON_FIELD_ERRORS = '__all__'
 
 class BaseDocument(object):
     __slots__ = ('_changed_fields', '_initialised', '_created', '_data',
-                 '_dynamic_fields', '_auto_id_field', '_db_field_map', '_cls', '__weakref__')
+                 '_dynamic_fields', '_auto_id_field', '_db_field_map', '__weakref__')
 
     _dynamic = False
     _dynamic_lock = True
@@ -78,6 +78,9 @@ class BaseDocument(object):
             value = getattr(self, key, None)
             setattr(self, key, value)
 
+        if "_cls" not in values:
+            self._cls = self._class_name
+
         # Set passed values after initialisation
         if self._dynamic:
             dynamic_data = {}

mongoengine/base/metaclasses.py

@@ -47,6 +47,10 @@ class DocumentMetaclass(type):
                     meta.merge(base._meta)
             attrs['_meta'] = meta
 
+        if '_meta' in attrs and attrs['_meta'].get('allow_inheritance', ALLOW_INHERITANCE):
+            StringField = _import_class('StringField')
+            attrs['_cls'] = StringField()
+
         # Handle document Fields
 
         # Merge all fields from subclasses