Finding embedded documents in Mongoose ODM

I have a model with array of embedded documents. That was made by design since we often (almost always) query root document together with embedded docs.

var SubsetSchema = new mongoose.Schema({
    number: {
        type: Number,
        index: true,
        unique: true,
    },
    name: String,
});

var RootSchema = new mongoose.Schema({
    name: String,
    subsets: [SubsetSchema],
});

mongoose.model('collection', RootSchema);
var Root = module.exports = mongoose.model('collection');

And finding a single subset document is not a problem using:

Root.findOne({'subsets.number': 3}, {_id: 0, 'subsets.$': 1}, ...);

However when i need to find multiple subset documents (and in our case using regex) doesn't seem to be possible:

Root.find({'subsets.name': /regex/i}, {_id: 0, 'subsets.$': 1}, ...);

It gives the following error:

error: {
    "$err" : "positional operator (subsets.$) requires corresponding field in query specifier",
    "code" : 16352
}

How would i do it in this case? Splitting Schema into two collections isn't an option, because that will destroy our performance on other more frequent queries.