Knockout.js Select value binding to object

Question

I am failing with Knockout select list binding when using an object as a select list value. It works fine if I use a string, but I want to bind objects.

I have a Gift object and it has a Title, Price and Company. I have a select list of companies and each company has an Id and Name. The initial selection however is not the correct in the select list.

Please see fiddle: http://jsfiddle.net/mrfunnel/SaepM/

This is important to me when binding to MVC3 view models. Though I admit it may be because I am doing things the wrong way.

If I have the following model:

public class Company
{
    public Guid Id { get; set; }
    public string Name { get; set; }

}
public class GiftModel
{
    public Company Company { get; set; }
    public string Title { get; set; }
    public double Price { get; set; }
}

How do I select a Company that is bindable in my controller? Do I need to add a CompanyId property to the GiftModel and bind to that or write custom binder. Am I missing something fundamental here?

Thanks in advance.

Answer 1

You need to do a lot of stuff.

An CompanyId in your ViewModel is the only way to bind and make this observable. You can not make a object observable only it´s values

<form class="giftListEditor" data-bind="submit: save">
    <!-- Our other UI elements, including the table and ‘add’ button, go here -->

    <p>You have asked for <span data-bind="text: gifts().length">&nbsp;</span> gift(s)</p>
    <table>
        <tbody  data-bind="foreach: gifts">
            <tr>
                <td>Gift name: <input data-bind="value: Title"/></td>
                <td>Price: $ <input data-bind="value: Price"/></td>
                <td>Company: <select data-bind="options: $root.companies, optionsText: 'Name', optionsValue: 'Id', value: CompanyId"/></td>
                <td>CompanyId: <span data-bind="text: CompanyId"></span></td>
                <td><a href="#" data-bind="click: $root.removeGift">Delete</a></td>
            </tr>
        </tbody>
    </table>
    <button data-bind="click: addGift">Add Gift</button>
    <button data-bind="enable: gifts().length > 0" type="submit">Submit</button>
</form>​

your model

// Fake data
var initialData = [
    { Title: ko.observable('Item 1'), Price: ko.observable(56), CompanyId: ko.observable(1) },
    { Title: ko.observable('Item 2'), Price: ko.observable(60), CompanyId: ko.observable(2) }, 
    { Title: ko.observable('Item 3'), Price: ko.observable(70), CompanyId: ko.observable(2) }
];

var initialCompanies = [
    { Id: 1, Name: "Comp 1" },
    { Id: 2, Name: "Comp 2" },
    { Id: 3, Name: "Comp 3" }
];

var viewModel = {
    gifts: ko.observableArray(initialData),
    companies: initialCompanies,

    addGift: function() {
        this.gifts.push({
            Title: "",
            Price: "",
            Company: { Id: "", Name: "" }
        });
    },
    removeGift: function($gift) {
        viewModel.gifts.remove($gift);
    },
    save: function() {
        console.log(ko.toJS(viewModel.gifts));
    }
};

ko.applyBindings(viewModel, document.body);​

Answer 2

To make an object observable, use the foeach binding. If you have such a scenario:

var model = {
    myObj : ko.observable();
}

if you try to bind to myObj.label it won't work:

<span><a href="#" data-bind="text: myObj.label"></a></span>

however, using the foreach binding:

<span data-bind="foreach: myObj"><a href="#" data-bind="text: label"></a></span>

ko iterates through the properties as it would through an array in the usual javascript manner and things will work.