Custom knockout binding handler to work with the Select2 project

gistfile1.js

//
// Assumes that you are binding to a hidden input element. 
//
// Bindings:
//   * select2 : passthrough options to the select2 javascript call
//   * selectedItem : object to initially select, or null
//   * itemMapper : defaaults to {id: 'id', text: 'text'}
//   * items : set of items to init the selection list with
//
// example:
//  <input type="hidden" data-bind="select2: {placeholder: 'Select me', allowClear : true}, 
//      selectedItem: selectme, items: clients mapper: {id: 'Id', text:'ClientName'} />
//
ko.bindingHandlers.select2 = {
    init: function (element, valueAccessor, allBindingsAccessor) {
        var obj = valueAccessor();
        var allBindings = allBindingsAccessor();
        var itemMapper = allBindings.itemMapper || {id : 'id', text: 'text'};
        obj.data = obj.data || {results: [], text: (itemMapper.text || 'text')};
        var items = ko.utils.unwrapObservable(allBindings.items);
        _(items).each(function (anitem) {
            var theid = anitem[itemMapper.id];
            var thetext = anitem[itemMapper.text];
            obj.data.results.push({ id: theid, text: thetext });
        });
        $(element).select2(obj);
        var selected = ko.utils.unwrapObservable(allBindings.selectedItem);
        if (null != selected) {
            $(element).select2("val", selected[itemMapper.id]);
        }
        ko.utils.domNodeDisposal.addDisposeCallback(element, function () {
            $(element).select2('destroy');
        });

        // keeps the backing ko model object in sync with select2 selection events
        $(element).on('change', function updateSelect2ObservableModel(val, added, removed) {
            var idkey = itemMapper.id;
            var newkey = $(val.target).select2('val');
            var items = ko.utils.unwrapObservable(allBindings.items);
            var match = _(items).find(function(iter) {
                return iter[idkey] == newkey;
            });
            allBindings.selectedItem(match);
        });
    },
    update: function (element, valueAccessor, allBindingsAccessor) {
        var obj = valueAccessor(),
            allBindings = allBindingsAccessor(),
            itemMapper = allBindings.itemMapper || { id: 'id', text: 'text' };
        var disable = allBindings.disable ? ko.utils.unwrapObservable(allBindings.disable) : false,
            enable = allBindings.enable ? ko.utils.unwrapObservable(allBindings.enable) : true;
        if (enable && !disable) {
            $(element).select2("enable");
        } else {
            $(element).select2("disable");
        }
        var selected = ko.utils.unwrapObservable(allBindings.selectedItem);
        if (! _(selected).isUndefined()) 
            $(element).select2("val", selected[itemMapper.id]);
        $(element).trigger('change');
    }
};