Dealing with subclasses

//I have a model class called 

Portal.Document = can.Model({},{});

//and a couple of subclass

Portal.WordDocument = Portal.Document({}, {});

Portal.ExcelDocument = Portal.Document({}, {});

// In my app I render a table of Portal.Documents (I fetch the model instances using findAll)

// This call will include documents of all types in one list, e.g. word and excel

Portal.Document.findAll()

.done(

      function(){ 

      renderDocuments(); 

});

// Here is where it gets tricky: As a user I want to now edit one document, e.g. if I wanted to edit the individual metadata (in a single page //application scenario). As it is a Word document I want to "cast" the generic Portal.Document instance as a Portal.WordDocument, in order //to make the subsequent code cleaner (more object oriented perhaps).

//At the moment I do this:

var wordDoc = new Portal.WordDocument(doc.serialize());

//However One of the main problems with this is that there are now two model instances representating the same object ( same row 

//in the DB table essentially). So when I update the attributes of one, the canJS magic does not automatically update the other.

Am I going about this the right way? I'm trying to avoid having a Portal.Document model class that has code that needs to handle all types of documents, e.g.

//I want to avoid this

if(word){

      do this

}else if(excel){

      do this

}

Am I going about this the right way?

To summarise, I want a generic findAll for documents, but would like to separate unique behaviour for different types into subclasses. But I also want the live binding to continue working, so that when the subclass is updated, so is the instance of the "generic" document.

Thanks in advance,

Bryon