Mongoose find vs exec. How to return values?

Question

I'm new programming in Node and I'm stuck because I no understand exactly what is the difference between .find and .exec. Here is the code that I'm trying to run:

exports.getPopulatedUsers = async function(query){
    var res = await users.find(query, function (err, docs) {
       if(err) return err;
       return docs;
   });
    return res; //  **Work fine, res return docs**
}

But when execute with .exec the res is undefined.

exports.getPopulatedUsers = async function(query){
    var res = await users.find(query).exec(function (err, docs) {
       if(err) return err;
       return docs;
   });
    return res; //  **ERROR, res is undefined **
}

What am I doing wrong?

I think this might help you https://stackoverflow.com/a/41148831/6511655 — Felix Fong, Jun 19 '18 at 16:23
This can be helpful too http://mongoosejs.com/docs/promises.html — iofjuupasli, Jun 19 '18 at 16:24
thanks, @iofjuupasli. Your link explains a lot about .exec and Promise in Mongodb — Thinh NV, Aug 17 '20 at 03:05

score 6 · Answer 1 · answered Jan 22 '19 at 16:01

find() and exec() serve two different purposes.
The find() method is a database READ operation enabling function. It is present in both the native mongodb driver for node, as well as the Mongoose library which internally uses the mongodb drivers and is especially useful for imposing a fixed schema.
Now, in mongodb driver, if you use find(query), the query is automatically executed where as in mongoose it wouldn't. We need helper functions/ callbacks to get the operation to execute. exec is one such helper function. It goes something like: myPlaylist.findOne({'singer':'Adam Levine'}).exec() Mongoose queries are not promises. They have a .then() function for convenience.
If you need a fully-fledged promise, use the .exec() function.

So, you could do a myPlaylist.findOne({'singer':'Adam Levine'}).then() but that would return a Mongoose/BlueBird (yet another library) promise and a typical JavaScript one.

Note: exec() takes an optional callback function. It's only when you do not use a callback that you get back a Promise.

So to get the fully-fledged promise i would do `myPlaylist.findOne({'singer':'Adam Levine'}).exec().then(...) ?` Or I must use the bluebird library for that? @sushmita-mallick — O'Dane Brissett, Sep 20 '19 at 15:56
Yes, it would return a fully fledged promise if you do it this way @O'DaneBrissett — Sushmita Mallick, Sep 26 '19 at 13:18

score 1 · Accepted Answer · answered Jun 19 '18 at 16:34

1

You can not mix promise with callbacks in mongoose:

exports.getPopulatedUsers = async function(query){
    var docs = await users.find(query).exec();
    return docs;
}

as for what's the difference between exec and find, read the links in the comment section they are very clear.

answered Jun 19 '18 at 16:34

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Would you handle errors with `.then()` `.catch()` ? – O'Dane Brissett Sep 20 '19 at 15:59
As OP explicitly asked what the difference is and you did not provide this answer (essentially saying "go look at the docs"), I don't believe this is a good answer. Please edit with the difference between the two. – dylanh724 Jun 14 '21 at 07:38

Mongoose find vs exec. How to return values?

2 Answers2