Question

classroom.manager can be null on the server. So it may not come from server. I want to add it and create with my own constructor if it does not come from server. If it comes from server I want to create it with it’s own data.

 var mappingOption = {

      create: function (classroom) {
        var res = ko.mapping.fromJS(clasroom.data);

        if (res.manager == undefined)
            res.manager = ko.observable(new Manager());

        return res;
    },
    'manager': {
        create: function (args) {
            var res = ko.mapping.fromJS(args.data);
            var res2 = new Manager();
            $.extend(res2, res)
            return res2;
        }
    }

Answer 1

Class room model

var Classroom = function(data) {
    self = this;
    data = data || {};
        self.manager = data.manager || new Manager();

    ko.mapping.fromJS(data, { 
        'manager': {
            function (managerData) {
                return new Manager(managerData);
                }
            }
        }, self);

};

Manager model

var Manager = function(data) {
    self = this;
    data = data || {};
    ko.mapping.fromJS(data, {}, self);
};

Key part is the data || new Manager() section