--- a/entity.py	Tue May 24 10:33:43 2011 +0200
+++ b/entity.py	Wed May 25 08:35:20 2011 +0200
@@ -62,6 +62,23 @@
     return True
 
 
+def remove_ambiguous_rels(attr_set, subjtypes, schema):
+    '''remove from `attr_set` the relations of entity types `subjtypes` that have
+    different entity type sets as target'''
+    for attr in attr_set.copy():
+        rschema = schema.rschema(attr)
+        if rschema.final:
+            continue
+        ttypes = None
+        for subjtype in subjtypes:
+            cur_ttypes = rschema.objects(subjtype)
+            if ttypes is None:
+                ttypes = cur_ttypes
+            elif cur_ttypes != ttypes:
+                attr_set.remove(attr)
+                break
+
+
 class Entity(AppObject):
     """an entity instance has e_schema automagically set on
     the class and instances has access to their issuing cursor.
@@ -222,6 +239,7 @@
                 if len(targettypes) > 1:
                     # find fetch_attrs common to all destination types
                     fetchattrs = user._cw.vreg['etypes'].fetch_attrs(targettypes)
+                    remove_ambiguous_rels(fetchattrs, targettypes, user._cw.vreg.schema)
                 else:
                     fetchattrs = etypecls.fetch_attrs
                 etypecls._fetch_restrictions(var, varmaker, fetchattrs,
@@ -739,6 +757,10 @@
         etypecls = self._cw.vreg['etypes'].etype_class(targettypes[0])
         if len(targettypes) > 1:
             fetchattrs = self._cw.vreg['etypes'].fetch_attrs(targettypes)
+            # XXX we should fetch ambiguous relation objects too but not
+            # recurse on them in _fetch_restrictions; it is easier to remove
+            # them completely for now, as it would require an deeper api rewrite
+            remove_ambiguous_rels(fetchattrs, targettypes, self._cw.vreg.schema)
         else:
             fetchattrs = etypecls.fetch_attrs
         rql = etypecls.fetch_rql(self._cw.user, [restriction], fetchattrs,