var express = require('express');
var app = express();
var bodyparser = require('body-parser');
var mongoose = require('mongoose');
books = require('./models/books.js');
mongoose.connect('mongodb://localhost/books');
var db = mongoose.connection;
app.get('/api/authors', function (req, res) {
books.getBooks(function (books,err) {
if(err){
throw err;
}
res.json(books);
});
});
Why we cannot use the function(err, books) as function(books, error).
I want to know what principle it violates.
When query is executed, results are passed as parameters to callback function. If If there is any error in executing the query, the error is passed as first argument and the results are passed as second parameter to the callback function. And this is how it works.
So, you can't use it interchangeably.
In your case books.getBooks(function (books,err) {.. if there is any error books will be the one containing in it. And if not, there will be results in err params.
And I assume your query is working OK and you are throwing error checking on err value that's why you see the error.
May be you are getting confused with the names of the params. Remember, they are just the variable names, results are there according to the position of variables in callback.
To answer your question on:
Why we cannot use the function(err, books) as function(books,
error).
Most npm modules follow the Continuation-passing style(CPS) design pattern, which uses:
cb(null, data) to pass on a successful result.
cb(err) to pass on an error and exit the function.
and, the function has only one outcome.
For example:
function getBooks(cb) {
let books, error;
// .... Perform the operations
// .... If all goes well store
// .... the results in books
if (books) {
cb(null, result);
} else {
error = "There was an error loading books"
cb(error)
}
}
This is not a complete example, but shows the essence of it.
TL;DR: That, my friend, is convention.
The only thing that I can think of without knowing the error is that you might want to use:
let books = mongoose.model ('Books');
And your model should be called Books.
Is it possible to include the Error message and Model so we could have some more info about the problem?
Related
userSchema.pre('save',async function(next){
//hash the password before saving user to database
next()
})
Hey guys I'm trying to understand the concept of middleware in mongoose. Assuming that I have an userSchema that I run the pre hook on to hash the password before saving the user to the database. On the surface, as far as I understand, the above code that I have will hash the password (not the important part for this question) and then call next() to signal that the function is done. However, I am trying to understand how things work under the hood. I want to know what is next() ? Can you guys walk me through an example of how everything works together under the hood from start to end once the code get executed or somehow help me to have a better understanding of this concept? Thanks
Short : with the pre method you can register listeners for certain events of your Schemas. So pre('save', callback) will fire whenever you save a document of said Model. pre means it will execute before the event, so it can be used (for example) to hash a password before saving it to the document.
However, you have several options to define them, see below :
The combination of using an async callback function and providing the next parameter is not necessary, you can either :
use normal callback with next parameter
the next parameter is a function provided to you by mongoose to have a way out, or to tell mongoose you are done and to continue with the next step in the execution chain. Also it is possible to pass an Error to next it will stop the execution chain.
schema.pre('save', function(next) {
// do stuff
if (error) { return next(new Error("something went wrong"); }
return next(null);
});
use async callback
Here the execution chain will continue once your async callback has finished. If there is an error and you want to break/stop execution chain you just throw it
schema.pre('save', async function() {
// do stuff
await doStuff()
await doMoreStuff()
if (error) { throw new Error("something went wrong"); }
return;
});
Straight from the docs : https://mongoosejs.com/docs/middleware.html#pre
Example
const { Schema, model } = require('mongoose');
const SomeSchema = new Schema ({
name : { type : String }
});
SomeSchema.pre('save', function (next) {
console.log('pre save fired!');
return next();
});
const Some = model('Some', SomeSchema);
console.log('before create');
const doc = new Some({ name : 'test' });
doc.save((err) => {
console.log('after saved');
});
This will output
before create
pre save fired!
after saved
I'm trying to query a collection of users using Mongoose in a Node API.
The handler looks like this:
exports.getUsers = async function(req, res, next) {
try {
let users = db.User.find();
return res.status(200).json(users);
} catch(e) {
return next(e);
}
};
This returns an error that reads Converting circular structure to JSON. When I console.log() the results of db.User.find(), I get a Query object. I've checked everything else. All of my other routes are working normally.
Well...I figured it out. I'll post the answer that I discovered in case anyone else is trying to figure this out. It turns out, through a little bit more careful reading of the documentation, that the Query object that is returned has to be executed. There are two ways to execute it - with a callback function or by returning a promise (but not both). I found this page on queries in the mongoose docs helpful. My final handler looked like this.
exports.getUsers = async function(req, res, next) {
try {
db.User.find()
.then(users => {
return res.status(200).json(users);
});
} catch(e) {
return next(e);
}
};
Next time I guess I'll dig around for a few more minutes before asking.
Edit to add:
Found a second solution. Due to the use of the async function, I also was able to use following inside the try block.
let users = await db.User.find();
return res.status(200).json(users);
i'm working on a Meteor project, and I must say that isn't easy at all, especially for one thing: callbacks !
Everything is async, so I wonder how do I must do to get results from my mongodb.
var user = Meteor.users.findOne({username: "john"});
return (user); // sometimes returns "undefined"
...
var user = Meteor.users.findOne({username: "john"});
if (user) // so ok, I check if it exists!
return (user); // Cool, I got my user!
return (); // Ok and what should I return here? I want my user!
I don't want to be dirty and put like setTimeout everywhere.
Anybody has a solution for this ?
EDIT :
I noticed in router.js with console.log that my data is returned 4 times. 2 times with an undefined value and 2 other times with the expected value. In the view, it's still undefined.
Why the router passes like 4 times in this route ? Does it display the first result of the return value in the router ?
What should I return if the find() doesn't find anything ?
EDIT 2: Here is some code to understand.
this.route('profilePage', {
path: 'profil/:_id?',
waitOn: function() {
return [
Meteor.subscribe('article', { prop: this.params._id}), // id can be id or username
Meteor.subscribe('article', { userId: this.params._id}), // id can be id or username
Meteor.subscribe('params'),
Meteor.subscribe('profil', (this.params._id ? this.params._id : Meteor.userId()))
];
},
data: function() {
if (this.params._id) {
var user = Meteor.users.findOne(this.params._id);
if (!user)
user = Meteor.users.findOne({username: this.params._id});
console.log(user);
return user;
}
else if (Meteor.userId())
return Meteor.user();
else
Router.go("userCreate");
}
});
I get this on the console:
http://puu.sh/debdJ/69419911f7.png
(text version following)
undefined
undefined
Object_id: "o3mgLcechYTtHPELh"addresses: (....)
Object_id: "o3mgLcechYTtHPELh"addresses: (....)
findOne(yourId) is a sync method which is equivalent to find({ _id: yourId}, callback). The difference is that find() allows you to define a callback. If you don't pass a callback to find() this method will be sync.
check wrapAsync: http://docs.meteor.com/#/full/meteor_wrapasync
It allows you to code in a sync style with a async operations.
Free lesson on EventedMind: https://www.eventedmind.com/feed/meteor-meteor-wrapasync
My experience thus far is that the Meteor Mongodb package is that the functions do not generally provide callbacks (for some reason insert does...), the functions are atomic (thus sync).
There are meteor packages that can make Mongodb async if you want (I havn't tried any).
I guess this sync approach is in line with the simple maintenance goal of Mongodb. Thinking about it, one of my pet peeves using Node is working with async callback waterfalls/nests, they are a pain to create and maintain... and hopefully this will make my code easier to read and understand and change...
var future = new Future();
var _h = Hunts.findOne({huntId});
if(_h) {
future.return(_h)
} else {
return future.wait();
}
on server/startup.js you need:
Future = Npm.require('fibers/future');
I am trying to send a request parameter through to an 'exports' method for a mongodb find in an express.js, backbone.js application. I am having a difficult
time getting the parameters to pass through to mongodb and with '#'.
The breakage is the passing of parameters into the exported mongodb function.
Here is the flow of data:
First the request is successfully routed to the 'upcoming' function:
"upcoming/uni/:uni" : "upcoming",
It flows on to the 'upcoming' function without a problem.
upcoming: function(uni) {
console.log("uni: "+uni);
pag.reset();
console.log("Hit upcoming list target");
setCollectionType('upcoming');
var upcomingCourses = buildCollection();
// ------------------------------------------------------------------------
// here is the problem how do I pass the parameter value through the fetch?
// Although it may also have to do with '#' please read on.
// ------------------------------------------------------------------------
upcomingCourses.fetch({success: function(){
$("#content").html(new ListView({model: upcomingCourses, page: 1}).el);
}});
this.headerView.selectMenuItem('home-menu');
},
The routing for the mongo methods is:
app.get('/upcoming/uni/:uni', mongomod.findUpcoming);
So the following method is exported from the mongodb js file and is executed reliable. However the req.params are not passed through.
Interspersed in the code I have described its' runtime behaviour:
exports.findUpcoming = function(req, res) {
console.log("university", req.params.uni); // This consistently is unpopulated
var uni = req.params.uni;
console.log("Size: "+req.params.length); // This will always be 0
for (var i=0; i < req.params.length; i++) {
console.log("Parameters: "+req.params[i]);
}
db.collection('upcoming', function(err, collection) {
if (typeof uni === 'undefined') {
console.log("The value is undefined");
uni = "Princeton University"; // here we add a string to test it it will work.
}
collection.find({university:uni}).toArray(function(err, items) {
if (err) {
console.log("Error: "+err);
} else {
console.log("No Error");
console.log("Count: "+items.length);
console.log(items[0]['university']);
res.send(items);
}
});
});
};
On additional and important note:
The url, in a working, runtime environment would be:
http://localhost:3000/#upcoming/uni/Exploratorium
This one fails, but the following URL will work in passing the params through these functions however it returns the JSON to the screen rather then
the rendered version:
http://localhost:3000/upcoming/uni/Exploratorium
The problem could be a miss understanding of # and templates. Please, if you see the error enlightenment would be greatly appreciated.
Nothing after the # gets passed to the server. See How to get hash in a server side language? or https://stackoverflow.com/a/318581/711902.
I found a solution to the problem of passing the parameters from the client side to the server side. By changing the url of the collection the parameters will be passed to the server side:
upcomingCourses.url = "/upcoming/uni/"+uni; // <-- here's the ticket where uni is param
upcomingCourses.fetch({success: function(){
$("#content").html(new ListView({model: upcomingCourses, page: 1}).el);
}});
This can be made more elegant but it is a way to pass the parameters on to the server.
Thanks
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?
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
});
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.
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).
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.
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...
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);
});