diff -r caef98aa4a98 -r 59220b704562 server/sources/__init__.py
--- a/server/sources/__init__.py	Thu Sep 17 19:38:04 2009 +0200
+++ b/server/sources/__init__.py	Fri Sep 18 11:24:37 2009 +0200
@@ -80,6 +80,11 @@
     # a reference to the instance'schema (may differs from the source'schema)
     schema = None
 
+    # multi-sources planning control
+    dont_cross_relations = ()
+    cross_relations = ()
+
+
     def __init__(self, repo, appschema, source_config, *args, **kwargs):
         self.repo = repo
         self.uri = source_config['uri']
@@ -177,6 +182,19 @@
             return wsupport
         return True
 
+    def may_cross_relation(self, rtype):
+        """return True if the relation may be crossed among sources. Rules are:
+
+        * if this source support the relation, can't be crossed unless explicitly
+          specified in .cross_relations
+
+        * if this source doesn't support the relation, can be crossed unless
+          explicitly specified in .dont_cross_relations
+        """
+        if self.support_relation(rtype):
+            return rtype in self.cross_relations
+        return rtype not in self.dont_cross_relations
+
     def eid2extid(self, eid, session=None):
         return self.repo.eid2extid(self, eid, session)