Asp.net MVC 3 - controller action doesn't see the JSON object

I implemented a small test that is based on Scott Guthrie article http://weblogs.asp.net/scottgu/archive/2010/07/27/introducing-asp-net-mvc-3-preview-1.aspx

But my controller action thinks that object passed to it is null, even though I used JSON.stringify on it and packed data to it - I followed the article exactly.

In my HTML

@using (Html.BeginForm("SomeAction", "ControllerName", FormMethod.Post, new { id = "FormID" })) {  

   
    <div class="editor-field">@Html.EditorFor(model => model.Date)</div>   
    <div class="editor-field">@Html.EditorFor(model => model.Id)</div>    
    <div class="editor-field">@Html.EditorFor(model => model.Name)</div>

      
    <input type="button" value="save" onclick="Save();" />
} 

In my model file

namespace MyNamespace.Models
{
    public class MyModel
    {
        public string Id { get; set; }
        public string DateReceived { get; set; }
        public string Name { get; set; }
    }
}

In my JS

function Save() {   
   
     var myModelObj = { Id: $("#Id").val(),
               DateReceived: $("#DateReceived").val(),
               Name: $("#Name").val()
    };


$.ajax({

    type: "POST",
    url: "/ControllerName/SomeAction",
    data: JSON.stringify(myModelObj),
    cache: false,
   //  dataType: "json",

    success: function (data) {

        alert('success!');
    },

    error: function (XMLHttpRequest, textStatus, errorThrown) {

        alert('error');
        alert(XMLHttpRequest + "<br />" + textStatus + "<br />" + errorThrown);

    }

});

In my controller

 [HttpPost]
    public ActionResult SomeAction(MyModel modelObj )
    {
       using (StreamWriter sw = new StreamWriter("/test.txt") )
        {

            sw.Write(modelObj.Id);
        } 
        return null;
    }

When I put a break point on sw.Write...statement, modelObj.Id is null.

Am I missing something or is there anything else that needs to be wired up?

I think you need to add the contentType and dataType options to your ajax call.

contentType: 'application/json, charset=utf-8',
dataType: 'json',