我一直在node.js中玩mongodb。我已经收集了一些数据(我知道已经检查过了)。当我尝试在集合上运行find()时,它返回未定义。我不知道为什么。代码如下:
function get_accounts(){
var MongoClient = mongodb.MongoClient;
var url = "url";
MongoClient.connect(url, function (err, db) {
if (err) {
console.log('Unable to connect to the mongoDB server. Error:', err);
} else {
//HURRAY!! We are connected. :)
console.log('Connection established to database');
var collection = db.collection('accounts');
collection.find().toArray(function(err, docs) {
console.log("Printing docs from Array")
docs.forEach(function(doc) {
console.log("Doc from Array ");
console.dir(doc);
});
});
console.log("mission complete");
}
db.close();
}
);
}
如果您知道为什么会这样,我想听听您的想法。谢谢!如果有任何区别,该数据库是mongolab托管的数据库。
由于node.js的异步特性,您将获得一个未定义的值,代码中的任何地方都没有逻辑告诉console.log语句等到find()
语句完成后再打印出文档。您必须了解Node.js中的回调的概念。但是,这里有一些问题可以解决。许多开始使用node的人倾向于嵌套很多匿名函数,从而创建了可怕的“厄运金字塔”或回调地狱。通过分解并命名一些功能,可以使它更简洁,易于使用:
var MongoClient = require("mongodb").MongoClient
// move connecting to mongo logic into a function to avoid the "pyramid of doom"
function getConnection(cb) {
MongoClient.connect("your-mongo-url", function(err, db) {
if (err) return cb(err);
var accounts = db.collection("accounts");
cb(null, accounts);
})
}
// list all of the documents by passing an empty selector.
// This returns a 'cursor' which allows you to walk through the documents
function readAll(collection, cb) {
collection.find({}, cb);
}
function printAccount(account) {
// make sure you found your account!
if (!account) {
console.log("Couldn't find the account you asked for!");
}
console.log("Account from Array "+ account);
}
// the each method allows you to walk through the result set,
// notice the callback, as every time the callback
// is called, there is another chance of an error
function printAccounts(accounts, cb) {
accounts.each(function(err, account) {
if (err) return cb(err);
printAccount(account);
});
}
function get_accounts(cb) {
getConnection(function(err, collection) {
if (err) return cb(err);
// need to make sure to close the database, otherwise the process
// won't stop
function processAccounts(err, accounts) {
if (err) return cb(err);
// the callback to each is called for every result,
// once it returns a null, you know
// the result set is done
accounts.each(function(err, account) {
if (err) return cb(err)
if (hero) {
printAccount(account);
} else {
collection.db.close();
cb();
}
})
}
readAll(collection, processAccounts);
})
}
// Call the get_accounts function
get_accounts(function(err) {
if (err) {
console.log("had an error!", err);
process.exit(1);
}
});
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