Viewmodel nested checkbox not binding on post - asp.net

Question

I have a subscription form that contains a matrix of options. The form can be seen in screenshot Subscription table
I am having trouble with ASP.NET MVC generating appropriate ID's and then on postback having the binder populate the model with the form selections.
The add on name is down the left side and when posted back the collection of SubscriptionInputModel.Addons get populated ok. But SubscriptionInputModel.Addons[i].SubscriptionLevelCombos is null as seen in debug screenshot
The current code is using CheckBoxFor but I've also tried manually generating ID's in format:
#Html.CheckBox("addon[" + a + "].SubscriptionLevelCombo[" + i + "].AddonSelected", addon.SubscriptionLevelCombos[i].AddonSelected)
Neither format has worked and also experimented while debugging but no luck. I would appreciate any ideas. Worst case I assume I would need to read the raw form collection?
I assume the level of nested object shouldn't matter as it is all object path notation and array indexes in html tag names?
Here are snippets of current code to help illustrate what exists.
View Models
public class SubscriptionInputModel
{
//other stuff to come
//....
//add on's, listed down left of table
public List<SubscriptionInputAddonModel> Addons;
}
public class SubscriptionInputAddonModel
{
public int Id { get; set; }
public string Name { get; set; }
public string Note { get; set; }
public List<SubscriptionInputAddonComboModel> SubscriptionLevelCombos { get; set; }
}
public class SubscriptionInputAddonComboModel
{
public int? Id { get; set; }
public decimal? AddonCost { get; set; }
public CostTimeUnitOption? CostTimeUnit { get; set; }
public bool? IsComplimentaryBySubscriptionLevel { get; set; }
public string ComboText { get; set; }
public bool AddonSelected { get; set; }
public int? AddonId { get; set; }
}
SubscriptionController
[Route("identity/subscription")]
// GET: Subscription
public ActionResult Index()
{
SubscriptionInputModel model = new SubscriptionInputModel();
ArrbOneDbContext db = new ArrbOneDbContext();
List<SubscriptionInputAddonModel> addons = Mapper.Map<Addon[], List<SubscriptionInputAddonModel>>(db.Addons.OrderBy(a => a.OrderPosition).ToArray());
model.Addons = addons;
foreach(var addon in model.Addons)
{
var addonCombos = db.Database.SqlQuery<SubscriptionInputAddonComboModel>(#"SELECT SLA.Id, AddonCost, CostTimeUnit, IsComplimentaryBySubscriptionLevel, ComboText, AddonId
FROM SubscriptionLevel L
LEFT OUTER JOIN SubscriptionLevelAddon SLA ON L.Id = SLA.SubscriptionLevelId AND SLA.AddonId = #p0
ORDER BY L.OrderPosition", addon.Id);
addon.SubscriptionLevelCombos = addonCombos.ToList();
}
return View(model);
}
[Route("identity/subscription")]
[ValidateAntiForgeryToken]
[HttpPost]
// POST: Subscription
public ActionResult Index(SubscriptionInputModel model)
{
ArrbOneDbContext db = new ArrbOneDbContext();
List<SubscriptionInputAddonModel> addons = Mapper.Map<Addon[], List<SubscriptionInputAddonModel>>(db.Addons.OrderBy(a => a.OrderPosition).ToArray());
model.Addons = addons;
//debug breakpoint to inspect returned model values
return View();
}
Index.cshtml
#model Identity_Server._Code.ViewModel.Subscription.SubscriptionInputModel
#{
ViewBag.Title = "Subscription";
}
#using (Html.BeginForm("Index", "Subscription", new { signin = Request.QueryString["signin"] }, FormMethod.Post))
{
#Html.ValidationSummary("Please correct the following errors")
#Html.AntiForgeryToken()
...
// ADD ONs ----------------------------------------------------------------------------------
#for (int a = 0; a < Model.Addons.Count; a++)
{
var addon = Model.Addons[a];
<tr>
<td class="text-left">#addon.Name
<div class="SubscriptionItemNote">#addon.Note
#Html.HiddenFor(m => m.Addons[a].Id)
</div>
</td>
#for (int i = 0; i < addon.SubscriptionLevelCombos.Count; i++)
{
<td>
#if (addon.SubscriptionLevelCombos[i].Id.HasValue)
{
if (addon.SubscriptionLevelCombos[i].AddonCost.HasValue && addon.SubscriptionLevelCombos[i].AddonCost.Value > 0)
{
#Html.Raw("<div>+ " + #addon.SubscriptionLevelCombos[i].AddonCost.Value.ToString("0.##") + " / " + #addon.SubscriptionLevelCombos[i].CostTimeUnit.Value.ToString() + "</div>")
}
else if (addon.SubscriptionLevelCombos[i].IsComplimentaryBySubscriptionLevel.HasValue && #addon.SubscriptionLevelCombos[i].IsComplimentaryBySubscriptionLevel.Value)
{
<span class="glyphicon glyphicon-ok"></span>
}
if (!string.IsNullOrEmpty(addon.SubscriptionLevelCombos[i].ComboText))
{
<div>#addon.SubscriptionLevelCombos[i].ComboText</div>
}
if (addon.SubscriptionLevelCombos[i].AddonCost.HasValue && addon.SubscriptionLevelCombos[i].AddonCost.Value > 0)
{
#Html.HiddenFor(m => m.Addons[a].SubscriptionLevelCombos[i].Id)
#Html.CheckBoxFor(m => m.Addons[a].SubscriptionLevelCombos[i].AddonSelected)
}
}
</td>
}
</tr>
}