Javascript mongodb检查是否存在值（node.js）

Im使用Node.js和Mongo（mongodb驱动程序）将项目添加到集合中。 我有一个html站点，它使用socket.io将信息传递到页面。我可以插入到数据库中，但当我尝试查看是否存在值时，我从Mongo得到一个奇数返回。我发送了一个名字，并试图把它放在客户集合中。 到目前为止，我得到的是：

socket.on('DB', function(msg)
    {
        if(msg.Status == "Read")
        {
            MongoClient.connect(url, function(err, db)  //connect to mongo
            {
                var collection = db.collection('Clients');  // get reference to the collection
                collection.find(({Name: msg.Name},{$exists: true}), function(err, doc) //find if a value exists
                {     
                    if(doc) //if it does
                    {
                        console.log(doc); // print out what it sends back
                    }
                    else if(!doc) // if it does not 
                    {
                        console.log("Not in docs");
                    }
                });
              db.close();
            });

            }
}

这让我想起：

{ connection: null,
  server: null,
  disconnectHandler:
   { s: { storedOps: [], storeOptions: [Object], topology: [Object] },
     length: [Getter] },
  bson: {},
  ns: 'DB.Clients',
  cmd:
   { find: 'DB.Clients',
     limit: 0,
     skip: 0,
     query: { '$exists': true },
     slaveOk: true,
     readPreference: { preference: 'primary', tags: undefined, options: undefined } },
  options:
   { skip: 0,
     limit: 0,
     raw: undefined,
     hint: null,
     timeout: undefined,
     slaveOk: true,
     readPreference: { preference: 'primary', tags: undefined, options: undefined },
     db:
      { domain: null,
        _events: {},
        _maxListeners: 10,
        s: [Object],
        serverConfig: [Getter],
        bufferMaxEntries: [Getter],
        databaseName: [Getter],
        options: [Getter],
        native_parser: [Getter],
        slaveOk: [Getter],
        writeConcern: [Getter] },
     disconnectHandler: { s: [Object], length: [Getter] } },
  topology:
   { domain: null,
     _events:
      { reconnect: [Function],
        timeout: [Object],
        error: [Object],
        close: [Object],
        destroy: [Object] },
     _maxListeners: 10,
     s:
      { options: [Object],
        callbacks: [Object],
        logger: [Object],
        state: 'connected',
        reconnect: true,
        reconnectTries: 30,
        reconnectInterval: 1000,
        emitError: true,
        currentReconnectRetry: 30,
        ismaster: [Object],
        readPreferenceStrategies: undefined,
        authProviders: [Object],
        id: 5,
        tag: undefined,
        disconnectHandler: [Object],
        wireProtocolHandler: {},
        Cursor: [Object],
        bsonInstance: {},
        bson: {},
        pool: [Object],
        serverDetails: [Object] },
     name: [Getter],
     bson: [Getter],
     wireProtocolHandler: [Getter],
     id: [Getter] },
  cursorState:
   { cursorId: null,
     documents: [],
     cursorIndex: 0,
     dead: false,
     killed: false,
     init: false,
     notified: false,
     limit: 0,
     skip: 0,
     batchSize: 1000,
     currentLimit: 0,
     transforms: undefined },
  callbacks: null,
  logger: { className: 'Cursor' },
  _readableState:
   { highWaterMark: 16384,
     buffer: [],
     length: 0,
     pipes: null,
     pipesCount: 0,
     flowing: false,
     ended: false,
     endEmitted: false,
     reading: false,
     calledRead: false,
     sync: true,
     needReadable: false,
     emittedReadable: false,
     readableListening: false,
     objectMode: true,
     defaultEncoding: 'utf8',
     ranOut: false,
     awaitDrain: 0,
     readingMore: false,
     decoder: null,
     encoding: null },
  readable: true,
  domain: null,
  _events: {},
  _maxListeners: 10,
  s:
   { maxTimeMS: null,
     numberOfRetries: 5,
     tailableRetryInterval: 500,
     currentNumberOfRetries: 5,
     state: 0,
     streamOptions: {},
     bson: {},
     ns: 'DB.Clients',
     cmd:
      { find: 'DB.Clients',
        limit: 0,
        skip: 0,
        query: [Object],
        slaveOk: true,
        readPreference: [Object] },
     options:
      { skip: 0,
        limit: 0,
        raw: undefined,
        hint: null,
        timeout: undefined,
        slaveOk: true,
        readPreference: [Object],
        db: [Object],
        disconnectHandler: [Object] },
     topology:
      { domain: null,
        _events: [Object],
        _maxListeners: 10,
        s: [Object],
        name: [Getter],
        bson: [Getter],
        wireProtocolHandler: [Getter],
        id: [Getter] },
     topologyOptions:
      { socketOptions: {},
        auto_reconnect: true,
        host: 'localhost',
        port: 27017,
        cursorFactory: [Object],
        reconnect: true,
        emitError: true,
        size: 5,
        disconnectHandler: [Object],
        bson: {},
        messageHandler: [Function],
        wireProtocolHandler: {} } },
  timeout: false,
  sortValue: undefined,
  readPreference: { preference: 'primary', tags: undefined, options: undefined } }

在我获取的cli中运行文档时，无论是否存在具有该名称的文档，都将始终返回

> db.Clients.find({Name: 'say what'}, {$exists: true})
{ "_id" : ObjectId("5519a66eb85de65c121182d9") }

这表明文档存在，如果我对不在mongo中的文档运行相同的cmd，它将不返回任何内容。 是否仍要在return语句中找到一个引用来说明文档是否存在？

MongoDB方法将a返回到与查询条件匹配的文档。因此，您在

console.log（doc）

中看到的实际上是返回的光标。当

find（）

方法“返回文档”时，该方法实际上是将光标返回到文档

您需要从

find（）

方法向结果游标添加一个方法，例如

var collection = db.collection('Clients');  // get reference to the collection
collection.find({Name: msg.Name}, {$exists: true}).toArray(function(err, doc) //find if a value exists
{     
    if(doc) //if it does
    {
        console.log(doc); // print out what it sends back
    }
    else if(!doc) // if it does not 
    {
        console.log("Not in docs");
    }
});

该方法返回一个数组，其中包含游标中的所有文档。该方法完全迭代游标，将所有文档加载到RAM中并耗尽游标。

此

（{Name:msg.Name}，{$exists:true}）

未达到预期效果。请尝试使用

查找（…）.toArray（函数（err，docs）{…}）

，并验证文档的长度。我不知道这是否有问题，但很多例子都在使用这种模式。