def meta_node_properties_update(ts_begin, ts_end)
      connection[meta_collection].find_one_and_update(
        {},
        {
          '$set' => {
            last_node_properties_update: {
              ts_begin: ts_begin.iso8601,
              ts_end:   ts_end.iso8601
            }
          }
        },
        { upsert: true }
      )
      @node_properties_update_timestamp = ts_begin
    end
def meta_node_properties_update(ts_begin, ts_end)
      connection[meta_collection].find_one_and_update(
        {},
        {
          '$set' => {
            last_node_properties_update: {
              ts_begin: ts_begin.iso8601,
              ts_end:   ts_end.iso8601
            }
          }
        },
        { upsert: true }
      )
      @node_properties_update_timestamp = ts_begin
    end
update or insert timestamps for node_properties_update