Функция не срабатывает динамически

function PassportViewModel() { 
  var self = this; 
  var foreignDocumentTypeList = " + @Html.Raw(Json.Encode(ViewBag.ForeighDocumentTypeList as int[])) + @"; 
  self.chosenIdentificationDoctype = ko.observable(); 
  self.isForeign = ko.computed(function() { 
    return foreignDocumentTypeList.indexOf(parseInt(self.chosenIdentificationDoctype())) > -1; 
  }); 
  self.isForeignCit = ko.computed(function() { 
    return ($('#IdentificationDoctype_Id').val() == 20); 
  }); 
};

<div class="col-md-2"> 
  <label data-bind="text:isForeign()?'Тип':'Серия'"></label> 
</div> 
<div class="col-md-2"> 
  @Html.TextBoxFor(m => m.PassportSerial, new { @class = "form-control" }) @Html.ValidationMessageFor(m => m.PassportSerial) 
</div> 
<div class="col-md-2" data-bind="style:{display:isForeignCit()?'block':'none'}"> 
  @Html.LabelFor(m => m.Belonging) 
</div> 
<div class="col-md-3" data-bind="style:{display:isForeignCit()?'block':'none'}"> 
  @Html.HiddenFor(m => m.Belonging.Id, new { @class = "form-control", @style = "border:0;padding:0;margin:0;" }) @Html.ValidationMessageFor(m => m.Belonging) 
</div>