state = state.merge(Immutable.fromJS({
    transactions: {
      list: [],
      byDate: {}
    }
  }))
  let cursor = Cursor.from(state, ['transactions'], newData => {
    notifyObservers(cursor);
  });


/// ....

    getTransactions: function(month, year) {
      if (month != null && year != null) {
        return cursor.get('byDate').get(dateKey(month, year)).valueOf() ||
          Immutable.List();
      }
      return cursor.get('list').valueOf();
    },