diff -r 480187dd66b3 -r 88577b10b31e schemas/base.py
--- a/schemas/base.py	Thu Feb 13 16:37:40 2014 +0100
+++ b/schemas/base.py	Wed Feb 25 10:33:41 2015 +0100
@@ -23,7 +23,7 @@
 from yams.buildobjs import (EntityType, RelationType, RelationDefinition,
                             SubjectRelation,
                             String, TZDatetime, Datetime, Password, Interval,
-                            Boolean)
+                            Boolean, UniqueConstraint)
 from cubicweb.schema import (
     RQLConstraint, WorkflowableEntityType, ERQLExpression, RRQLExpression,
     PUB_SYSTEM_ENTITY_PERMS, PUB_SYSTEM_REL_PERMS, PUB_SYSTEM_ATTR_PERMS)
@@ -164,6 +164,7 @@
     cardinality = '11'
     subject = '*'
     object = 'String'
+    constraints = [UniqueConstraint()]
 
 
 # XXX find a better relation name
@@ -183,7 +184,6 @@
     cardinality = '?*'
 
 
-
 class ExternalUri(EntityType):
     """a URI representing an object in external data store"""
     uri = String(required=True, unique=True, maxsize=256,