code: def GetCollectionNode(self, partition_key):
          
          if partition_key is None:
              raise ValueError("partition_key is None or empty.")
  
          partition_number = self._FindPartition(self._GetBytes(partition_key))
          return self.partitions[partition_number].GetNode()
docs: Gets the SelfLink/ID based link of the collection node that maps to the partition key
          based on the hashing algorithm used for finding the node in the ring.
  
          :param str partition_key:
              The partition key to be used for finding the node in the ring.
  
          :return:
              The name of the collection mapped to that partition.
          :rtype: str