无法使用node.js和MongoDB包查询MongoDB数据库

我无法使用node.js和MongoDB包查询我的MongoDB数据库

我成功地插入了两个对象，如下所示：

var mongo = require("mongodb");
var host = "127.0.0.1";
var port = mongo.Connection.DEFAULT_PORT;
var db = new mongo.Db("nodejs-introduction", new mongo.Server(host, port, { }), {safe: true});
console.log(host);
db.open(function(error) {
    console.log("We are connected ", + host + ':' + port);

    db.collection("user", { w:1 }, function(error, collection){
        console.log("We have the collection");

        collection.insert({
            id: "1",
            name: "Foo Bar",
            twitter: "Foo_Bar",
            email: "info@nfoobar.com"
        }, {w:1}, function() {
            console.log("Succesfully inserted Foo Bar");
        });

        collection.insert({
            id: "2",
            name: "Bar Foo",
            twitter: "Bar_Foo",
            email: "info@tbarfoo.com"
        }, {w:1}, function() {
            console.log("Succesfully inserted Bar Foo");
        });
    });
});

var mongo = require("mongodb");
var host = "127.0.0.1";
var port = mongo.Connection.DEFAULT_PORT;
var db = new mongo.Db("nodejs-introduction", new mongo.Server(host, port, { }), {safe: false});
console.log(host);
db.open(function(error) {
    console.log("We are connected ", + host + ':' + port);

    //collection is sort of table
    db.collection("user", function(error, collection){
        console.log("We have the collection");
        collection.find({ "id": "1" }, function(error, cursor){
            cursor.toArray(function(error, users){
                if (users.length == 0) {
                    console.log("no user found");
                } else {
                    console.log("Found a user", users[0]); 
                }
            });
        });

    });
});

这里一切顺利，没有错误。但如果我想这样查询项目：

var mongo = require("mongodb");
var host = "127.0.0.1";
var port = mongo.Connection.DEFAULT_PORT;
var db = new mongo.Db("nodejs-introduction", new mongo.Server(host, port, { }), {safe: true});
console.log(host);
db.open(function(error) {
    console.log("We are connected ", + host + ':' + port);

    db.collection("user", { w:1 }, function(error, collection){
        console.log("We have the collection");

        collection.insert({
            id: "1",
            name: "Foo Bar",
            twitter: "Foo_Bar",
            email: "info@nfoobar.com"
        }, {w:1}, function() {
            console.log("Succesfully inserted Foo Bar");
        });

        collection.insert({
            id: "2",
            name: "Bar Foo",
            twitter: "Bar_Foo",
            email: "info@tbarfoo.com"
        }, {w:1}, function() {
            console.log("Succesfully inserted Bar Foo");
        });
    });
});

var mongo = require("mongodb");
var host = "127.0.0.1";
var port = mongo.Connection.DEFAULT_PORT;
var db = new mongo.Db("nodejs-introduction", new mongo.Server(host, port, { }), {safe: false});
console.log(host);
db.open(function(error) {
    console.log("We are connected ", + host + ':' + port);

    //collection is sort of table
    db.collection("user", function(error, collection){
        console.log("We have the collection");
        collection.find({ "id": "1" }, function(error, cursor){
            cursor.toArray(function(error, users){
                if (users.length == 0) {
                    console.log("no user found");
                } else {
                    console.log("Found a user", users[0]); 
                }
            });
        });

    });
});

然后我得到以下错误：

127.0.0.1
We are connected  NaN:27017
We have the collection

/path/to/query.js:14
                if (users.length == 0) {
                         ^
TypeError: Cannot read property 'length' of null

---

所以我理解的是变量users为null，它找不到任何匹配项，但我不知道为什么。我做错了什么？

找到了解决办法。Mongodb没有运行。如果MongoDB没有运行，我怎么能在没有任何错误的情况下插入MongoDB？

文档中可能没有元素

users

，因此返回

null

。您必须检查

null

：如果（users==null）{…}否则…没有匹配项，因此它返回

null

，而不是一个空的

数组

。我理解这一点，但我不理解的是为什么没有匹配项。请尝试从mongo shell进行查询。