Flutter中从Firestore文档提取数组转为对象列表的方法

我的Firestore文档中存储着一个notes数组，每个note包含作者、笔记文本等信息，我用Note类定义这些笔记。现在想从文档中提取notes数组并转换成List<Note>类型的对象列表，但当前代码运行异常——获取完整FilingData对象后提取的notesList为null。想知道正确实现方式，以及是否必须获取完整FilingData，还是可以直接拉取notes数组。

相关代码与结构 #

Note数据结构 #

Note类及转换器 #

class Note {
  String? filingId;
  final String name;
  final String note; 
  final String writerId;
  Timestamp? noteDate;
  
  Note ({
    required this.filingId,
    required this.name,
    required this.writerId,
    required this.note,
    required this.noteDate,
  });

  Map<String, dynamic> toFirestore() {
    return {
      'name': name,
      'writerId': writerId,
      'note': note,
      'noteDate': noteDate
    };
  }

  factory Note.fromFirestore(
    DocumentSnapshot<Map<String, dynamic>> snapshot,
    SnapshotOptions? options,) {
      final data = snapshot.data();
      return Note(
        noteDate: data?['noteDate'], 
        name: data?['name'],
        writerId: data?['writerId'], 
        note: data?['note'], 
        filingId: snapshot.id);
    }
}

FilingData类及转换器 #

class FilingData {
  Timestamp? createDate;
  String? filingId;
  final String jurisdiction; 
  final String filingName;
  String? frequency;
  Timestamp? nextDueDate;
  final String method;
  String? autoAssign;
  List<dynamic>? notes;
  Timestamp? updateDate;
  
  FilingData ({
    required this.createDate,
    filingId,
    required this.jurisdiction,
    required this.filingName,
    required this.frequency,
    required this.nextDueDate,
    required this.method,
    required this.autoAssign,
    notes,
    required this.updateDate,
  });    

factory FilingData.fromFirestore(
        DocumentSnapshot<Map<String, dynamic>> snapshot,
        SnapshotOptions? options,) {
          final data = snapshot.data();
          return FilingData(
            filingId: snapshot.id,
            createDate: data?['createDate'], 
            jurisdiction: data?['jurisdiction'],
            filingName: data?['filingName'], 
            frequency: data?['frequency'],
            nextDueDate: data?['nextDueDate'], 
            method: data?['method'],
            autoAssign: data?['autoAssign'],
            updateDate: data?['updateDate'],
            notes: data?['notes'] is Iterable ? List.from(data?['notes']) : null);
        }
}

当前出错代码 #

Future<List<Note>> getAllNotes(String filingId) async {
    var ref =  _db.collection('filings').doc(filingId).withConverter(
      fromFirestore: FilingData.fromFirestore, 
      toFirestore: (FilingData, _) => FilingData.toFirestore(),
    );

    final docSnap = await ref.get();
    var notesList = docSnap.data()?.notes;
    List<Note> allNotes = [];
    // for (int i = 0; i < notesList!.length; i++) {
    //   Note thisNote = Note(name: notesList[i].name, appUserId: notesList[i].appUserId, note: notesList[i].note, noteDate: notesList[i].noteDate);
    //   allNotes.add(thisNote);
    // }
    return allNotes;
}

一、修复通过FilingData获取并转换notes的代码 #

当前问题核心是FilingData.fromFirestore仅将notes转为List<dynamic>，未转换成Note对象；同时原注释代码存在字段错误（appUserId应为writerId）。

步骤1：修改FilingData的notes类型与转换器 #

将FilingData中的List<dynamic>? notes改为List<Note>? notes，并在fromFirestore中完成嵌套对象的转换：

class FilingData {
  // 其他字段保持不变
  List<Note>? notes; // 更新类型

  FilingData ({
    // 其他参数保持不变
    this.notes,
    required this.updateDate,
  });    

factory FilingData.fromFirestore(
        DocumentSnapshot<Map<String, dynamic>> snapshot,
        SnapshotOptions? options,) {
          final data = snapshot.data();
          // 将notes数组中的每个Map转换为Note对象
          final notesData = data?['notes'] as List?;
          final List<Note>? notes = notesData?.map((noteMap) {
            return Note(
              filingId: snapshot.id,
              name: noteMap['name'],
              writerId: noteMap['writerId'],
              note: noteMap['note'],
              noteDate: noteMap['noteDate'],
            );
          }).toList();

          return FilingData(
            filingId: snapshot.id,
            createDate: data?['createDate'], 
            jurisdiction: data?['jurisdiction'],
            filingName: data?['filingName'], 
            frequency: data?['frequency'],
            nextDueDate: data?['nextDueDate'], 
            method: data?['method'],
            autoAssign: data?['autoAssign'],
            updateDate: data?['updateDate'],
            notes: notes,
          );
        }
}

步骤2：更新getAllNotes函数 #

现在可以直接获取已转换好的List<Note>：

Future<List<Note>> getAllNotes(String filingId) async {
    var ref =  _db.collection('filings').doc(filingId).withConverter(
      fromFirestore: FilingData.fromFirestore, 
      toFirestore: (filingData, _) => filingData.toFirestore(), // 避免变量名与类名冲突
    );

    final docSnap = await ref.get();
    final filingData = docSnap.data();
    // 返回空列表避免null，也可根据业务需求返回null
    return filingData?.notes ?? [];
}

二、直接拉取notes数组（无需获取完整FilingData） #

如果不需要文档中的其他字段，可通过Firestore的字段路径仅获取notes字段，减少数据传输：

Future<List<Note>> getAllNotes(String filingId) async {
    final docSnap = await _db.collection('filings').doc(filingId)
        .get(const GetOptions(source: Source.serverAndCache));
    
    final notesData = docSnap.data()?['notes'] as List?;
    if (notesData == null) return [];

    // 转换为List<Note>
    return notesData.map((noteMap) {
      return Note(
        filingId: filingId,
        name: noteMap['name'],
        writerId: noteMap['writerId'],
        note: noteMap['note'],
        noteDate: noteMap['noteDate'],
      );
    }).toList();
}

优化补充：给Note添加fromMap工厂方法 #

为了让嵌套对象转换更清晰，可给Note类添加一个从Map转换的工厂方法：

class Note {
  // 其他代码不变
  factory Note.fromMap(Map<String, dynamic> map, String filingId) {
    return Note(
      filingId: filingId,
      name: map['name'],
      writerId: map['writerId'],
      note: map['note'],
      noteDate: map['noteDate'],
    );
  }
}

之后转换时可直接调用Note.fromMap(noteMap, filingId)，代码可读性更强。

内容的提问来源于stack exchange，提问作者TM Lynch