diff -r 81fce01b4cc0 -r e7514e949ad8 cubicweb/entity.py
--- a/cubicweb/entity.py	Thu Feb 18 09:24:43 2016 +0100
+++ b/cubicweb/entity.py	Thu Feb 25 09:28:21 2016 +0100
@@ -547,7 +547,7 @@
             self.e_schema, self.eid, list(self.cw_attr_cache), id(self))
 
     def __lt__(self, other):
-        raise NotImplementedError('comparison not implemented for %s' % self.__class__)
+        return NotImplemented
 
     def __eq__(self, other):
         if isinstance(self.eid, integer_types):