Mongoose - how to use model inheritance in a sub document collection

Question

I have this model setup. I want a parent record with an array of sub docs. The sub docs have a schema, and use inheritance.

//
// Children

function abstractSchema() {
    Schema.apply(this, arguments);

    this.add({
        name: String
    });
};

util.inherits(abstractSchema, Schema);

var mySchema = new abstractSchema();

// 
// Inherited Types 

var textPropertySchema = new abstractSchema({
    length: Number
});

var numberPropertySchema = new abstractSchema({
    dp: Number
});

//
// Parent Model

var myModelSchema = mongoose.Schema({
    name: String,
    properties : [mySchema]   
});

When i save each an instance of numberPropertySchema or textPropertySchema, the _t (type is written) and is able to deserialise properly.

When however added as a sub doc array, they're all persisted with the base object properties only.

Is there any way round this? any extensions that could be used?

Thanks sam