Knockout two way Binding for Dropdrown - binding

Question

I am facing the issue while binding the value for dropdown from a observable array.
Model
function question(item)
{
var self = this;
self.questionId = ko.observable(item.questionId || "");
self.position = ko.observable(item.position || "");
self.position.subscribe(function (data) {
// Some Logic
}, this, 'beforeChange');
api.position.subscribe(function (data) {
// Some Logic
}
}
View Model
function viewmodel(item)
{
var self = this;
self.questionList = ko.observableArray([]);
self.questionEntity = ko.observable(new question(item));
// initially Item will be blank
self.addQuestion = function(data)
{
data.position(self.questionList().length + 1);
self.questionList.push(data);
}
}
View
<div>
question Id : <input type="text" data-bind="value: questionEntity().questionId " >
<button data-bind='click: $root.addquestion'>Add Question</button>
</div>
<table>
<tbody data-bind='foreach: questionList '>
question Id : <span data-bind='text:questionId' ></span>
position :
<select data-bind='options: $root.positionList,optionsText: "id",optionsValue: "id" ,value: position'></select>
</tbody>
</table>
Problem
I have one popup div(addQuestion) and I will add the question. and I am keep on adding that question to my observable Array(questionList)
When i add First question my value in observable array is
questionId | Position
1 | 1
When I add second record
questionId | Position
1 | 1
2 | 2
When I Add thrid record, all the previous records POSITION value(value is binded to dropdown) is being reset to 1 except last record
questionId | Position
1 | 1
2 | 1
3 | 3
fourth record
questionId | Position
1 | 1
2 | 1
3 | 1
4 | 4
after debugging I found out its problem with two way binding with Dropdown
<select data-bind='options: $root.positionList,optionsText: "id",optionsValue: "id" ,value: position'></select>
in above code if change the binding(single way) like below it works fine
<select data-bind='options: $root.positionList,optionsText: "id",optionsValue: "id" ,value: position()'></select>
But I need to subscribe an event for the dropdown.