code: def to_barrier_key(cls, barrier_index_key):
      
      barrier_index_path = barrier_index_key.to_path()
  
      # Pick out the items from the _BarrierIndex key path that we need to
      # construct the _BarrierRecord key path.
      (pipeline_kind, dependent_pipeline_id,
       unused_kind, purpose) = barrier_index_path[-4:]
  
      barrier_record_path = (
          pipeline_kind, dependent_pipeline_id,
          _BarrierRecord.kind(), purpose)
  
      return db.Key.from_path(*barrier_record_path)
docs: Converts a _BarrierIndex key to a _BarrierRecord key.
  
      Args:
        barrier_index_key: db.Key for a _BarrierIndex entity.
  
      Returns:
        db.Key for the corresponding _BarrierRecord entity.