How to return Mongoose results from the find metho

问题:

Everything I can find for rending a page with mongoose results says to do it like this:

users.find({}, function(err, docs){
    res.render('profile/profile', {
        users:     docs
    });
});

How could I return the results from the query, more like this?

var a_users = users.find({}); //non-working example

So that I could get multiple results to publish on the page?

like:

/* non working example */
var a_users    = users.find({});
var a_articles = articles.find({});

res.render('profile/profile', {
      users:    a_users
    , articles: a_articles
});

Can this be done?

回答1:

You're trying to force a synchronous paradigm. Just does't work. node.js is single threaded, for the most part -- when io is done, the execution context is yielded. Signaling is managed with a callback. What this means is that you either have nested callbacks, named functions, or a flow control library to make things nicer looking.

https://github.com/caolan/async#parallel

async.parallel([
   function(cb){
      users.find({}, cb);
   },
   function(cb){
      articles.find({}, cb);
   }
], function(results){
   // results contains both users and articles
});

回答2:

I'll play the necromancer here, as I still see another, better way to do it.

Using wonderful promise library Bluebird and its promisifyAll() method:

var Promise = require('bluebird');
var mongoose = require('mongoose');

Promise.promisifyAll(mongoose); // key part - promisification

var users, articles; // load mongoose models "users" and "articles" here

Promise.props({
    users: users.find().execAsync(),
    articles: articles.find().execAsync()
  })
  .then(function(results) {
    res.render('profile/profile', results);
  })
  .catch(function(err) {
    res.send(500); // oops - we're even handling errors!
  });

Key parts are as follows:

Promise.promisifyAll(mongoose);

Makes all mongoose (and its models) methods available as functions returning promises, with Async suffix (.exec() becomes .execAsync(), and so on). .promisifyAll() method is nearly-universal in Node.JS world - you can use it on anything providing asynchronous functions taking in callback as their last argument.

Promise.props({
    users: users.find().execAsync(),
    articles: articles.find().execAsync()
  })

.props() bluebird method takes in object with promises as its properties, and returns collective promise that gets resolved when both database queries (here - promises) return their results. Resolved value is our results object in the final function:

• results.users - users found in the database by mongoose
• results.articles - articles found in the database by mongoose (d'uh)

As you can see, we are not even getting near to the indentation callback hell. Both database queries are executed in parallel - no need for one of them to wait for the other. Code is short and readable - practically corresponding in length and complexity (or rather lack of it) to wishful "non-working example" posted in the question itself.

Promises are cool. Use them.

回答3:

The easy way:

var userModel = mongoose.model('users');
var articleModel = mongoose.model('articles');
userModel.find({}, function (err, db_users) {
  if(err) {/*error!!!*/}
  articleModel.find({}, function (err, db_articles) {
    if(err) {/*error!!!*/}
    res.render('profile/profile', {
       users: db_users,
       articles: db_articles
    });
  });
});

Practically every function is asynchronous in Node.js. So is Mongoose's find. And if you want to call it serially you should use something like Slide library.

But in your case I think the easiest way is to nest callbacks (this allows f.e. quering articles for selected previously users) or do it completly parallel with help of async libraries (see Flow control / Async goodies).

回答4:

I have a function that I use quite a bit as a return to Node functions.

function freturn (value, callback){
    if(callback){
        return callback(value); 
    }
    return value; 
}; 

Then I have an optional callback parameter in all of the signatures.

回答5:

I was dealing with a very similar thing but using socket.io and DB access from a client. My find was throwing the contents of my DB back to the client before the database had a chance to get the data... So for what it's worth I will share my findings here:

My function for retrieving the DB:

//Read Boards - complete DB

var readBoards = function() {
        var callback = function() {
            return function(error, data) {
                if(error) {
                    console.log("Error: " + error);
                }
                console.log("Boards from Server (fct): " + data);

            }
        };

        return boards.find({}, callback());
    };

My socket event listener:

socket.on('getBoards', function() {
        var query = dbConnection.readBoards();
        var promise = query.exec();
        promise.addBack(function (err, boards) {
            if(err)
                console.log("Error: " + err);
            socket.emit('onGetBoards', boards);
        });
    });

So to solve the problem we use the promise that mongoose gives us and then once we have received the data from the DB my socket emits it back to the client...

For what its worth...

回答6:

You achieve the desired result by the following code. Hope this will help you.

var async = require('async');

// custom imports
var User = require('../models/user');
var Article = require('../models/article');

var List1Objects = User.find({});
var List2Objects = Article.find({});
var resourcesStack = {
    usersList: List1Objects.exec.bind(List1Objects),
    articlesList: List2Objects.exec.bind(List2Objects),
};

async.parallel(resourcesStack, function (error, resultSet){
    if (error) {
        res.status(500).send(error);
        return;
    }
    res.render('home', resultSet);
});