First attempt at creating a custom observable for Knockout to wrap a NestedTypes model instance attribute efficiently.

NetedTypesObservable.js

	function modelAttributeObservable(model, attribute)
	{
		// Check the attribute exists
		if (!(attribute in model)) {
			throw new ReferenceError(attribute + ' is not an attribute of the supplied model');
		}
		
		// Setup the attribute observable cache
		model._koObservables || (model._koObservables = {});
	
		// If we already have a cached observable then just return it		
		if (attribute in model._koObservables) {
			return model._koObservables[attribute];
		}
	
		// Create our observable getter/setter function	
		function observableAttribute()
		{
			if (arguments.length > 0) {
				observableAttribute.valueWillMutate();
				model[attribute] = arguments[0];
				return this;
			} else {
				// The caller only needs to be notified of changes if they did a "read" operation
				ko.computedContext.oc(observableAttribute);
				return observableAttribute.peek();
			}
		}

		// Create a function to fetch the attribute value that
		// is in the correct context so it has access to the model.
		observableAttribute.peek = function() {
			return model[attribute];
		}

		// Register to listen to change events on the model attribute
		model.on('change:'+attribute, function() {
			observableAttribute.valueHasMutated();
		});
			
		// Initialise the observableAttribute as a subscribable
		// Then override the prototype to make it a subclass
		ko.subscribable.call(observableAttribute);
		observableAttribute.__proto__ = observableAttribute_fn;
			
		// setup the deferred update extender if needed
		ko.options['deferUpdates'] && ko.extenders['deferred'](observableAttribute, true);
		
		// Cache the observableAttribute function and return it
		return (model._koObservables[attribute] = observableAttribute);
	};

	
	// Setup the prototype for the observableAttribute as a subclass of subscribable
	observableAttribute_fn = {
		__proto__: ko.subscribable['fn'],
		valueHasMutated: function () { this.notifySubscribers(this.peek()); },
		valueWillMutate: function () { this.notifySubscribers(this.peek(), 'beforeChange');},

		// Make KO treat the observableAttribute function as an observable
		'__ko_proto__': ko.observable
	};