C# MVC Post Model and additional data to controller from js

Question

I am working with a custom workflow solution that I am creating. I would like to create a postback that has the model, and two integer values that represent the action and step that I have completed. I don't want to add them to the model because they are only used in this one place. The signature for this postback would be something like this.

[HttpPost]
public void ProcessWorkflowAction(EditScreenModelValidation model, int stepActionId, int stepNumber)
{
    //Some work on model and actions
}

I would really like to do this through JS because that is currently how I am getting StepActionId and StepId. Is there a way to package up the model to send through JS?

 var modelObj = CreateModelData();
 var postObj = JSON.stringify(modelObj);
 $.ajax({
        type: "POST",
        traditional: true,
        dataType: "json",
        url: "url",
        data: { model: modelObj, stepActionId: stepId, stepNumber: 3 }
        cache: false,
        complete: function (data) {
        }});

CreateModelData = function () {
    var modelObj = {};
    var modelArray = $('#frm').serializeArray()
    $.each(modelArray, function (index, value) {
        assign(modelObj, value.name, value.value);
    })

    return modelObj;
};

function assign(obj, prop, value) {
    if (prop != undefined) {
        if (typeof prop === "string")
            prop = prop.split(".");

        if (prop.length > 1) {
            var e = prop.shift();
            assign(obj[e] =
                     Object.prototype.toString.call(obj[e]) === "[object Object]"
                     ? obj[e]
                     : {},
                   prop,
                   value);
        } else
            obj[prop[0]] = value;
    }
}

The model comes back as null in the controller. I have also tried the following code with the same result.

$.ajax({
        type: "POST",
        traditional: true,
        dataType: "json",
        url: "url",
        data: { model: $('#frm').serializeArray(), stepActionId: stepId, stepNumber: 3 }
        cache: false,
        complete: function (data) {
        }});

Answer 1

You need to build up the object, assign the properties (make sure it matches any model validation and the field names are the same as your model) and use JSON.stringify to make the conversion:

 var modelObj = {};
 modelObj.prop1 = $('#txtField1').val();
 modelObj.prop2 = $('#txtField2').val();
 // etc... make sure the properties of this model match EditScreenModelValidation

 var postObj = JSON.stringify(modelObj); // convert object to json

 $.ajax({
    type: "POST",
    traditional: true,
    dataType: "json",
    url: "/Workflow/Home/ProcessWorkflowAction",
    data: { model: postObj, stepActionId: stepId, stepNumber: 3 }
    cache: false,
    complete: function (data) {
        if (data.responseText.length > 0) {
            var values = $.parseJSON(data.responseText)
            $('#ActionErrors').html(values.message)
        }
        else {
            location.reload();
        }
    }});