diff -r 3c281b6f16c6 -r 8d246203730a entities/adapters.py
--- a/entities/adapters.py	Thu Jun 10 16:28:04 2010 +0200
+++ b/entities/adapters.py	Thu Jun 10 16:28:06 2010 +0200
@@ -179,10 +179,16 @@
     __regid__ = 'ITree'
     __select__ = implements(ITree) # XXX for bw compat, else should be abstract
 
-    tree_relation = None
     child_role = 'subject'
     parent_role = 'object'
 
+    @property
+    def tree_relation(self):
+        warn('[3.9] tree_attribute is deprecated, define tree_relation on a custom '
+             'ITree for %s instead' % (self.entity.__class__),
+             DeprecationWarning)
+        return self.entity.tree_attribute
+
     @implements_adapter_compat('ITree')
     def children_rql(self):
         """returns RQL to get children