Document dict access now only looks for fields

mongoengine/base.py

@@ -202,8 +202,10 @@ class BaseDocument(object):
         """Dictionary-style field access, return a field's value if present.
         """
         try:
+            if name in self._fields:
                 return getattr(self, name)
         except AttributeError:
+            pass
         raise KeyError(name)
 
     def __setitem__(self, name, value):