Fix backup restore

This merge request fixes the backup restore system.

Note that it might not be backward compatible.

Fixes #492

beat/web/experiments/models/experiment.py

@@ -278,9 +278,9 @@ class Experiment(Shareable):
     #_____ Utilities __________
 
     def natural_key(self):
-
         return (self.author.username,) + \
             self.toolchain.natural_key() + (self.name,)
+    natural_key.dependencies = ['toolchains.toolchain']
 
 
     #_____ Methods __________