ko.mapping create function, extend object

http://stackoverflow.com/questions/9779969/ko-mapping-create-function-extend-object

HTML

<script src="http://knockoutjs.com/downloads/knockout-2.0.0.debug.js"></script>
<script src="https://rawgithub.com/SteveSanderson/knockout.mapping/master/build/output/knockout.mapping-latest.debug.js"></script>

JavaScript

var data =   [{ id: 0, child: { prop1 : 'a', prop2 : 'b' } },{ id: 0, child: null } ];


var viewModel = {
    data: ko.mapping.fromJS([]),
};

var mappingOptions = {
    child: {
        create: function(options) {
            if (!options.data)
                return null;
            return ko.mapping.fromJS(options.data);
        }
    }
}

ko.mapping.fromJS(data, mappingOptions, viewModel.data);   

$.each(viewModel.data(), function(){
     console.log(this.child)
})