code: func (m *metadataTracker) fromDB(db *instance, folder []byte) error {
  	key := db.keyer.GenerateFolderMetaKey(nil, folder)
  	bs, err := db.Get(key, nil)
  	if err != nil {
  		return err
  	}
  	return m.Unmarshal(bs)
  }
docs: // fromDB initializes the metadataTracker from the marshalled data found in
  // the database under the key corresponding to the given folder