mongodb - 两次调用Future后出现NoSuchMethodError

Question

我有一个简单的方法，可以从MongoDB中获取一些数据:

import 'package:mongo_dart/mongo_dart.dart';

Future find() {
  return _db.open().then((_) {
    return _db.collection('foo').find().toList();
  }).then((val) {
    _db.close();
    return val;
  });
}

如果我一次调用此方法，它将起作用。连续调用两次将生成 NoSuchMethodError: method not found: 'query'。

谁能解释这个问题？

这是完整的堆栈跟踪:

Uncaught Error: The null object does not have a method 'query'.

NoSuchMethodError: method not found: 'query'
Receiver: null
Arguments: [Instance of 'MongoQueryMessage']
Stack Trace: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      Db.queryMessage (package:mongo_dart/src/database/db.dart:174:28)
#2      Cursor.nextObject (package:mongo_dart/src/database/cursor.dart:68:29)
#3      Cursor._nextEach (package:mongo_dart/src/database/cursor.dart:102:15)
#4      Cursor.forEach (package:mongo_dart/src/database/cursor.dart:122:14)
#5      Cursor.toList (package:mongo_dart/src/database/cursor.dart:128:24)
#6      DbConnection.find.<anonymous closure> (package:schafkopfer_server/src/schafkopfer/data.dart:49:52)
#7      _rootRunUnary (dart:async/zone.dart:730)
#8      _RootZone.runUnary (dart:async/zone.dart:864)
#9      _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:488)
#10     _Future._propagateToListeners (dart:async/future_impl.dart:571)
#11     _Future._complete (dart:async/future_impl.dart:317)
#12     Future.forEach.nextElement (dart:async/future.dart:303)
#13     _rootRunUnary (dart:async/zone.dart:730)
#14     _RootZone.runUnary (dart:async/zone.dart:864)
#15     _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:488)
#16     _Future._propagateToListeners (dart:async/future_impl.dart:571)
#17     _Future._completeWithValue (dart:async/future_impl.dart:331)
#18     _Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:393)
#19     _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:23)
#20     _asyncRunCallback (dart:async/schedule_microtask.dart:32)
#21     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:128)

我打开了 issue for mongo_dart。

Answer 1

我相信您在混合异步代码和同步代码，并且在大多数情况下不起作用。

方法find返回Future，但是您尝试在同步模式下第二次调用find。
您是否会从find子句中第二次调用then，您的代码将起作用。

main() {
  var mongoTest = new MongoTest();
  mongoTest.find().then((x) {
    print('first find $x');
    mongoTest.find().then((x) => print('second find $x'));
  });
}

因此mongo_dart Db实例可以在关闭后打开，但在该示例中几乎没有意义。

如果您尝试在Web堆栈的服务器端进行操作，则建议您查看现有框架。例如红石。

http://redstonedart.org/2014/07/14/managing-database-connection-with-redstondart/