.

lib/zodb.py

@@ -182,11 +182,17 @@ def zconn_at(zconn): # -> tid
 
     # ZODB4
     #
-    # We rely on our patch in 4-nxd branch.
+    # We rely on our patch in 4-nxd branch that reworks ZODB.Connection to
+    # implement MVCC via always calling loadBefore(zconn._txn_time) to load objects.
     elif zmajor == 4:
-        assert 'conn:at' in ZODB.nxd_patches    # XXX error message if not there
-        return zconn.at()
+        assert 'conn:MVCC-via-loadBefore-only' in ZODB.nxd_patches
 
+        if zconn._mvcc_storage:
+            raise NotImplementedError("Connection.at for IMVCCStorage is not implemented")
+        if zconn.before is not None:
+            return before2at(zconn.before)      # historical connection
+        else:
+            return before2at(zconn._txn_time)   # "current" connection
 
     # ZODB3
     else: