code: def link(self, source_key, target_key):
      '''Creates a symbolic link key pointing from `target_key` to `source_key`'''
      link_value = self._link_value_for_key(source_key)
  
      # put straight into the child, to avoid following previous links.
      self.child_datastore.put(target_key, link_value)
  
      # exercise the link. ensure there are no cycles.
      self.get(target_keyf link(self, source_key, target_key):
      '''Creates a symbolic link key pointing from `target_key` to `source_key`'''
      link_value = self._link_value_for_key(source_key)
  
      # put straight into the child, to avoid following previous links.
      self.child_datastore.put(target_key, link_value)
  
      # exercise the link. ensure there are no cycles.
      self.get(target_key)
docs: Creates a symbolic link key pointing from `target_key` to `source_key`