Does MongoDB's $in clause guarantee order

Question

When using MongoDB\'s $in clause, does the order of the returned documents always correspond to the order of the array argument?

Answer 1

If you don't want to use aggregate, another solution is to use find and then sort the doc results client-side using array#sort:

If the $in values are primitive types like numbers you can use an approach like:

var ids = [4, 2, 8, 1, 9, 3, 5, 6];
MyModel.find({ _id: { $in: ids } }).exec(function(err, docs) {
    docs.sort(function(a, b) {
        // Sort docs by the order of their _id values in ids.
        return ids.indexOf(a._id) - ids.indexOf(b._id);
    });
});

If the $in values are non-primitive types like ObjectIds, another approach is required as indexOf compares by reference in that case.

If you're using Node.js 4.x+, you can use Array#findIndex and ObjectID#equals to handle this by changing the sort function to:

docs.sort((a, b) => ids.findIndex(id => a._id.equals(id)) - 
                    ids.findIndex(id => b._id.equals(id)));

Or with any Node.js version, with underscore/lodash's findIndex:

docs.sort(function (a, b) {
    return _.findIndex(ids, function (id) { return a._id.equals(id); }) -
           _.findIndex(ids, function (id) { return b._id.equals(id); });
});