Angular Drop-down inside of ng-repeat with additional logic needed for selected option

Question

This is basically a 2 part question which is why some of the other answers I've found on StackOverflow don't sufficiently cover what I'm asking.

Also need to note: using Bootstrap.
My simplified html can be explained like this:

<div class="container-fluid">
   <div class="row"> Headers </div>
   <div class="ParentObjectDiv" ng-repeat="pobj in pobjlist">//pobj list is returned from call in controller
       <div class="row"> ParentObject values </row>
       <div class="ChildObjectContainer">
          <div class="row"> Headers </div>
          <div class="ChildObjectDiv" ng-repeat="cobj in pobj.cobjs">
             <div class="row">
                <div class="col-md-1">
                   <select ng-model="cobj.someproperty"
                           ng-options="option for option in optionlist">
 //Here is the issue : the optionlist is returned from a separate call in the constructor
 //The cobj.someproperty can be null, and when that's the case, I want a custom selection
 //that says please select a property
                   </select>
                </div>
             </div>
          </div>
       </div>
    </div>
 </div>

Ok, so on to what I have tried.

1. Can't do a selectedOption = "whatever" in the controller for obvious reasons.

2. I have tried adding an angular expression that sets the property of cobj.someproperty to a value like "Value not know" if it detects that the property is empty/null.

   This is probably coming down to a fundamental misunderstanding of angular on my part. I have tried putting the expression that assigns this "not known" value to the property in various expression directives and just in the html itself. It is not an option to have this "not known" value on the actual object in the database and the list of Pobj's and their Cobj's is very large so I don't think I can spare iterating the entire list structure to set it before rendering and then iterating it again in the ng-repeats. I'm hoping that I can have a quick expression that evaluates null and sets it. Something like:

   {{cobj.someproperty = cobj.someproperty ? cobj.someproperty : "value not known"}}
   

   But not sure if that's a valid expression and if it is where I would put it because so far I've tried putting it in the ng-model itself, an ng-if, and just a blank line of html all within the scope of the cobj in the ng-repeat and none are working.

3. I haven't "tried" this, but my preferred solution and the one I've been researching is to see if there is a way to instead of evaluating and changing the cobj.someproperty, to change the ng-model of the select to point at a dummy property and at the same time set the cobj.dummyproperty to cobj.someproperty if some property is set, otherwise to set it to "value not known".

Additional considerations :
The parentobject list can be very large (>1k) and has on average 3 childobjects (but can have anywhere from 1 to 20) so performance is an issue.
There will be many cases where the user will not want to set this property so "value not known" when null will not be a temporary thing.
A user also needs to be able to select "value not known" if they have later realize they were mistake about the cobj.someproperty.
I am going to work on the logic to push the changes back to the database later so I am not worried about how the value is stored in the model in angular because I will most likely not use the ng-model to update the field. Right now I'm just worried with the selected option of the drop down.

Answer 1

Here is the answer with simplified html.

JavaScript code

var app = angular.module('plunker', []);

    app.controller('MainCtrl', function($scope) {
        $scope.items = [{
            "someProperty": "abc"
        }, {
            "someProperty": "xyz"
        }, {
            "someProperty": null
        }]

        $scope.optionlist = ["abc","xyz","pqr"];
    });

HTML code:

<table ng-controller="MainCtrl" >
<tbody>
<tr ng-repeat="item in items track by $index">
    <td width="75%">
        {{ item.someProperty }}
    </td>
    <td width="25%">
        <select ng-model="item.someProperty" >
        <option value="">Value not know</option>
        <option value="{{option}}" ng-repeat="option in optionlist">{{option}}</option>
        </select>
    </td>
</tr>
</tbody>

So whenever angular finds a null value for someProperty it binds the value to "Value Not Know". I am using ngRepeat instead of ngOption. Let me know if this is what you are looking for.

here is my plunk