Manual subscriptions in Knockout.js

code.js

// Unit.fetch makes an ajax call and passes new Unit(data) to the callback.
Unit.fetch(options, function(unit){
    addFunctions(unit)
    App.VMs.unit(unit)
}

// the console never logs "subscribed"
var addFunctions = function(viewModel){
   viewModel.units.subscribe(function(){
        console.log("subscribed")
        var units = this.units();
        for (var i = 0, j = units.length; i < j; i++) {
           var unit = units[i];
            if (!unit.index) {
               unit.index = ko.observable(i);  
            } else {
               unit.index(i);   
            }
        }
    }, viewModel);
}

model.js

var Unit = function(data){
    var _this = this


    this.root_unit = function(){
        return this.id() == App.VMs.course().root_unit_id()
    } 


    this.show_children = function(){
        //console.log(this.nested_children())
        return this.nested_children()
    }

    var mappings = {
        'units' : {
            key: function(item) {
                return ko.utils.unwrapObservable(item.id);
            }
            ,create: function(options){
                return new Unit(options.data) 
            }
        }

        ,'contents' :  {
            create : function(options){
                return new Content(options.data)
            }
        }
        
    } 

    this.save = function(){
        var data = {}
        data._timestamp = new Date().getTime();
        data.username = App.VMs.course().username()
        data.id = this.id()
        data.course_slug = App.VMs.course().slug()
        data['unit']  = this.toJSON()
        amplify.request(
            'unit#update'
            , data
        )
    }

    this.destroy = function(context){
        amplify.request(
            "unit#delete"
            ,{ 
                username : App.VMs.course().username()
                ,course_slug : App.VMs.course().slug()
                ,id : this.id()
            }
        )
        context.remove(this)
    }


    this.addChild = function(){
        var _this = this
        var data = { username : App.VMs.course().username()
                    ,course_slug : App.VMs.course().slug()
                    ,parent_id : _this.id 
        }
        data._timestamp = new Date().getTime();
        amplify.request(
            "unit#create"
            ,data
            ,function(data){
                data._timestamp = new Date().getTime();
                caches.units[data.id] = data
                unit = new Unit(data)
                _this.units.push(unit)
            })
    } 

    ko.mapping.fromJS(data, mappings, _this)

    this.dirtyFlag = new ko.dirtyFlag(_this, false)
}