Question

I am working through a KnockOutJs Sample and am having some posting problems with MVC3. My sample correctly posts when using a full page postback. When I try to save using an jQuery Ajax post, I can see the Post in the using Firebug NET viewer to be:

{ gifts:[{“GiftId”:0,”Title”:”sad”,”Price”:3}] }

When I view the ControllerContext in the ModelBinder in MVC3, the form parameters are empty and the json does not bind. Any ideas as to what is happening?

I have tried a number of configurations but here is the jQuery posting code (currently hard-coded to a static value):

…

            $.ajax({
        url: "/Home/PartialUpdate", 
        type: 'POST', 
        cache: false,
        data:   '{ gifts:[{"GiftId":0,"Title":"sad","Price":3}] }', //ko.toJSON({ gifts: this.gifts }),   
        dataType: 'json' ,
        contentType: "application/json;",
        success: function(result){
            alert(result);
            var data = ko.utils.parseJson(result); 
            this.gifts =  ko.observableArray(data) ;
        },
        error:function(xhr,err){ 
            alert("readyState: " + xhr.readyState+"\nstatus: "+xhr.status); 
            alert("responseText: " + xhr.responseText);
        }
        });

---

Edit: Here is the MVC3 action code for the Ajax update code

[HttpPost]
public JsonResult PartialUpdate ([FromJson] IEnumerable<Gift> gifts)
{
    gifts = gifts ?? new List<Gift>();
    using (var context = new KnockOutContext())
    {
        // Add record if not in DB
        foreach (var gift in gifts )
        {
            context.Entry(gift).State = (gift.GiftId == 0) ? EntityState.Added : EntityState.Modified;
        }

        // Delete records if not in ViewModel
        foreach (var dbGift in context.Gifts)
        {
            if (gifts.SingleOrDefault(c => c.GiftId == dbGift.GiftId) == null)
                context.Gifts.Remove(dbGift);
        }
        context.SaveChanges();
    }
    return GetGifts_Json();
}

And the full postback code that works (from Steve Sanderson’s example at
http://blog.stevensanderson.com/2010/07/12/editing-a-variable-length-list-knockout-style/)

[HttpPost]
public ActionResult Index([FromJson] IEnumerable<Gift> gifts)
{
    SaveGifts(gifts);
    return RedirectToAction("Index");
}

using this custom model binder:

        public class FromJsonAttribute : CustomModelBinderAttribute
        {
            private readonly static JavaScriptSerializer serializer = new JavaScriptSerializer();

            public override IModelBinder GetBinder()
            {
                return new JsonModelBinder();
            }

            private class JsonModelBinder : IModelBinder
            {
                public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
                {
                    var stringified = controllerContext.HttpContext.Request[bindingContext.ModelName];
                    if (string.IsNullOrEmpty(stringified))
                        return null;
                    return serializer.Deserialize(stringified, bindingContext.ModelType);
                }
            }
        }

Answer 1

My guess is that you started with Steve’s download, which is using MVC2. MVC2 did not have the JsonValueProvider registered by default. The [FromJson] attribute was intended to work with URL-encoded JSON that was submitted via ko.utils.postJson (full postback). This is not necessary when posting JSON via AJAX with the correct content-type (in MVC3).

So, the easiest thing to do is upgrade your project to MVC 3 (easy way here) and remove the [FromJson] attribute from your partial update.

Working copy here.

One other really minor thing: your static data is currently invalid JSON ('{ gifts:[{"GiftId":0,"Title":"sad","Price":3}] }'). gifts would need to be "gifts"