Access ViewModel in Knockout Mapping Plugin

A possible solution is to use this {deferEvaluation: true}option. So your computed function will only be evaluated when it is viewModelready and your Totalproperty has actually been used :

var mapping = {
    'Collection': {
        create: function(options) {
            var model = ko.mapping.fromJS(options.data);
            model.Total = ko.computed(function() {
                var result = this.Price() * viewModel.Count();
                return result;
            }, model, {deferEvaluation: true});
            return model;
        }
    }
};

Demo JSFiddle .

However, with this approach, you are tightly coupling the view model name with the mapping options. Because if you change var viewModel = ko.mapping.fromJS(json, mapping);and name it viewModeldifferently, you must also update the mapping configuration.

Since it may not be possible to move the "parent" chain in the mapping configuration in this case, the mapping plugin is not the best solution...

Here's an alternative to using a hand-written view model and passing the "root" where needed:

var MainViewModel = function (json) {    
    ko.mapping.fromJS(json, { 'ignore': ["Foo"] }, this); //map the rest
    this.Foo = new FooViewModel(json.Foo, this);
}

var FooViewModel = function (json, root) {
    ko.mapping.fromJS(json, { 'ignore': ["Bar"] }, this); //map the rest
    this.Bar = new BarViewModel(json.Bar, root);
}

var BarViewModel = function (json, root) {
    ko.mapping.fromJS(json, { 'ignore': ["Collection"] }, this); //map the rest
    this.Collection = ko.mapping.fromJS(json.Collection, {
       create: function(options) {
            var model = ko.mapping.fromJS(options.data);
            model.Total = ko.computed(function() {
                var result = this.Price() * root.Count();
                return result;
            }, model);

            return model;

       }});
}

Demo JSFiddle .