diff -r dd969de519a8 trytond/trytond/model/modelsql.py
--- a/trytond/trytond/model/modelsql.py	Mon Nov 28 15:54:24 2016 +0100
+++ b/trytond/trytond/model/modelsql.py	Mon Jan 30 22:32:48 2017 +0100
@@ -304,10 +304,10 @@
                     cls.raise_user_error('foreign_model_missing',
                         error_args=error_args)
         for name, _, error in cls._sql_constraints:
-            if name in str(exception):
+            if name in exception.message:
                 cls.raise_user_error(error)
         for name, error in cls._sql_error_messages.iteritems():
-            if name in str(exception):
+            if name in exception.message:
                 cls.raise_user_error(error)

     @classmethod