javascript - mongodb 检查值是否存在(node.js)

Question

我正在使用 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 将信息传递给页面。我可以插入到数据库中，但是当我尝试查看一个值是否存在时，我从 Mongo 得到了一个奇怪的返回。我发送了一个名字并尝试将其放入客户集合中。 到目前为止我所拥有的是:

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();
            });

            }
}

这让我回来了:

{ 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 } }

当我在我得到的 cli 中运行它时，无论是否存在具有名称的文档，都会始终返回

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

这表明该文档存在，如果我对不在 mongo 中的文档运行相同的 cmd，它什么也不会返回。 有没有办法在 return 语句中找到一个引用，说明文档是否存在？

Answer 1

MongoDB find()方法返回 cursor匹配查询条件的文档。所以您在 console.log(doc) 中看到的实际上是返回的游标。当 find() 方法“返回文档”时，该方法实际上是将游标返回到文档。

您需要添加一个 toArray()从 find() 方法生成的游标的方法，例如

var collection = db.collection('Clients');  // get reference to the collection
collection.find({Name: msg.Name}, {$exists: true}).toArray(function(err, docs) //find if documents that satisfy the criteria exist
{     
    if(docs.length > 0) //if exists
    {
        console.log(docs); // print out what it sends back
    }
    else // if it does not 
    {
        console.log("Not in docs");
    }
});

toArray()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。

编辑:docs 是一个长度为 0 的数组。因此请检查数组的长度