Mongoose schema within schema

How can I add a schema to another schema? This doesn't seem to be valid:

var UserSchema = new Schema({
    name        : String,
    app_key     : String,
    app_secret  : String
})



var TaskSchema = new Schema({
    name            : String,
    lastPerformed   : Date,
    folder          : String,
    user            : UserSchema
})

I checked the website and it shows how to declare it for an array but not for single.

Thanks