Sorting by virtual field in mongoDB (mongoose)

Let's say I have some Schema which has a virtual field like this

var schema = new mongoose.Schema(
{
    name: { type: String }
},
{
    toObject: { virtuals: true },
    toJSON: { virtuals: true }
});

schema.virtual("name_length").get(function(){
    return this.name.length;
});

In a query is it possible to sort the results by the virtual field? Something like

schema.find().sort("name_length").limit(5).exec(function(docs){ ... });

When I try this, the results are simple not sorted...