Question

I found an example here to create a select list with optgroups using KnockoutJS. This works fine, but I want to bind the value of the dropdown to my own javascript object, then access a particular property of that object:

<select data-bind="foreach: groups, value:selectedOption">
    <optgroup data-bind="attr: {label: label}, foreach: children">
        <option data-bind="text: label"></option>
    </optgroup>
</select>

function Group(label, children) {
    this.label = ko.observable(label);
    this.children = ko.observableArray(children);
}

function Option(label, property) {
    this.label = ko.observable(label);
    this.someOtherProperty = ko.observable(property);
}

var viewModel = {
    groups: ko.observableArray([
        new Group("Group 1", [
            new Option("Option 1", "A"),
            new Option("Option 2", "B"),
            new Option("Option 3", "C")
        ]),
        new Group("Group 2", [
            new Option("Option 4", "D"),
            new Option("Option 5", "E"),
            new Option("Option 6", "F")
        ])
    ]),
    selectedOption: ko.observable(),
    specialProperty: ko.computed(function(){
       this.selectedOption().someOtherProperty();
    })
};

ko.applyBindings(viewModel);

​

Answer 1

A good choice for this situation is to create a quick custom binding that let’s your “hand-made” options behave in the same way as options created by the options binding (attaches the object as meta-data). The binding could simply look like:

ko.bindingHandlers.option = {
    update: function(element, valueAccessor) {
       var value = ko.utils.unwrapObservable(valueAccessor());
       ko.selectExtensions.writeValue(element, value);   
    }        
};

You would use it like:

<select data-bind="foreach: groups, value: selectedOption">
    <optgroup data-bind="attr: {label: label}, foreach: children">
        <option data-bind="text: label, option: $data"></option>
    </optgroup>
</select>

Sample here: http://jsfiddle.net/rniemeyer/aCS7D/