如何将Map<String,List<dynamic>>转为Map<String,List<DetailedPositions>>？解决类型异常

问题概述 #

从PostgreSQL读取记录并按cust_id层级分组，尝试将Map<String, List<dynamic>>转换为Map<String, List<DetailedPositions>>时，抛出异常：'_PostgreSQLResultRow' is not a subtype of type 'Map<String, dynamic>'。

错误原因 #

_fetchMasterPositionData!.toList()返回的是List<_PostgreSQLResultRow>类型集合，而非List<Map<String, dynamic>>。直接将_PostgreSQLResultRow实例传入DetailedPositions.fromJson()会报错，因为该工厂方法要求参数为Map<String, dynamic>类型。

解决方案 #

方案1：直接从ResultRow构造对象（推荐） #

跳过fromJson方法，直接从_PostgreSQLResultRow中提取字段值构造DetailedPositions实例，避免Map转换步骤：

核心修正代码：

groupedPositions.forEach((key, value) {
  newMap[key] = value.map((e) => DetailedPositions(
    custID: e[0].toString(),
    custName: e[1].toString(),
    quantity: e[2] as int,
  )).toList();
});

方案2：将ResultRow转为Map后适配fromJson #

如果必须使用fromJson方法，需先将_PostgreSQLResultRow转为对应的Map<String, dynamic>，注意数据库字段名与fromJson中key的对应关系：

groupedPositions.forEach((key, value) {
  newMap[key] = value.map((e) => DetailedPositions.fromJson({
    'custID': e[0],
    'custName': e[1],
    'quantity': e[2],
  })).toList();
});

若使用的postgres库支持ResultRow.toColumnMap()方法（返回字段名到值的映射），可调整fromJson适配数据库字段名后直接转换：

// 修改DetailedPositions.fromJson适配数据库字段名
factory DetailedPositions.fromJson(Map<String, dynamic> json) {
  return DetailedPositions(
    custID: json['cust_id'] as String,
    custName: json['cust_name'] as String,
    quantity: json['net_quantity'] as int,
  );
}

// 转换逻辑
groupedPositions.forEach((key, value) {
  newMap[key] = value.map((e) => DetailedPositions.fromJson(e.toColumnMap())).toList();
});

完整修正后代码 #

Future<Map<String, List<DetailedPositions>>> fetchDetailedPositions(
    String custID) async {
  await connection!.open();
  Map<String, List<DetailedPositions>> newMap = {};
  await connection!.transaction((fetchDataConn) async {
    final _fetchMasterPositionData = await fetchDataConn.query(
      "SELECT cust_id, cust_name, net_quantity FROM positions");

    if (_fetchMasterPositionData.affectedRowCount > 0) {
      final fetchDataFuture = _fetchMasterPositionData.toList(growable: true);
      final Map<String, List<dynamic>> groupedPositions = groupBy(
          fetchDataFuture, (item) => item[0].toString() + ',' + item[1]);

      groupedPositions.forEach((key, value) {
        newMap[key] = value.map((e) => DetailedPositions(
          custID: e[0].toString(),
          custName: e[1].toString(),
          quantity: e[2] as int,
        )).toList();
      });
    }
  });
  return newMap;
}

class DetailedPositions {
  String custID;
  String custName;
  int quantity;

  DetailedPositions({
    required this.custID,
    required this.custName,
    required this.quantity,
  });

  // 若使用方案2，可保留或修改此方法
  factory DetailedPositions.fromJson(Map<String, dynamic> json) {
    return DetailedPositions(
      custID: json['custID'] as String,
      custName: json['custName'] as String,
      quantity: json['quantity'] as int,
    );
  }
}

内容的提问来源于stack exchange，提问作者anilraj