Multiple populates - mongoosejs

The best solution in my opinion is arrays when you are populating more than one foreign field on the same level. My code shows that I have multiple populates for different levels.

const patients = await Patient.find({})
                    .populate([{
                        path: 'files',
                        populate: {
                            path: 'authorizations',
                            model: 'Authorization'
                        },
                        populate: {
                            path: 'claims',
                            model: 'Claim',
                            options: {
                                sort: { startDate: 1 }
                            }
                        }
                    }, {
                        path: 'policies',
                        model: 'Policy',
                        populate: {
                            path: 'vobs',
                            populate: [{
                                path: 'benefits'
                            }, {
                                path: 'eligibility', 
                                model: 'Eligibility'
                            }]
                        }
                    }]);

As you can see, wherever I needed more than one field of a document populated, I encased the populate key in an array and provided an array of objects, each object having a different path. Most robust and concise way to do it, in my opinion.

You're already using the correct syntax of:

OrderModel.find()
    .populate('user')
    .populate('meal')
    .exec(function (err, results) {
         // callback
    });

Perhaps the meal ObjectId from the order isn't in the Meals collection?

UPDATE:
This solution remains for the version 3.x of Mongoose
http://mongoosejs.com/docs/3.8.x/docs/populate.html
but is no longer documented for >= 4.x versions of Mongoose and so the answer from @JohnnyHK is the only valid one for now on.

ORIGINAL POST
If you're using Mongoose >= 3.6, you can pass a space delimited string of the path names to populate:

OrderModel.find()
    .populate('user meal')
    .exec(function (err, results) {
         // callback
    });

http://mongoosejs.com/docs/populate.html

This has probably been resolved already, but this is my take on multiple & deep population in Mongodb > 3.6:

OrderModel.find().populate([{
    path: 'user',
    model: 'User'
}, {
    path: 'meal',
    model: 'Meal'
}]).exec(function(err, order) {
    if(err) throw err;
    if(order) {
        // execute on order
        console.log(order.user.username); // prints user's username
        console.log(order.meal.value);    // you get the idea
    }
});

There are probably other ways to do this, but this makes very readable code for beginners (like me)