Error: [ngModel:nonassign] Expression is non-assignable - angularjs

Question

Trying to display a columnvalue from a gridcollection based on another value in that same row.
The user can select/change values in a modal which contains a grid with values. When the modal closes the values are passed back. At that moment I would like to set a value for 'Also known as':
html:
Also known as: <input type="text" `ng-model="displayValue(displayNameData[0].show,displayNameData[0].value)">`
I created a function on scope to select the value only when the 'show' value is true:
$scope.displayValue = function (show, val) {
if (show) {
return val;
}
else {
return '';
}
}
However when I close the modal I get an error:
Error: [ngModel:nonassign] Expression 'displayValue(displayNameData[0].show,displayNameData[0].value)' is non-assignable.
plnkr reference:http://plnkr.co/edit/UoQHYwAxwdvX0qx7JFVW?p=preview

Answer 1

Using ng-value instead of ng-model worked for me.

Answer 2

As HackedByChinese mentioned, you can't bind ng-model to a function, so try like this:
<input type="text" ng-if="displayNameData[0].show"
ng-model="displayNameData[0].value">
Or if you want this control to be visible you can create directive, add function to $parsers that will set empty value according to show:
angular.module('yourModule').directive('bindIf', function() {
return {
restrict: 'A',
require: 'ngModel',
link: function(scope, element, attrs, ngModel) {
function parser(value) {
var show = scope.$eval(attrs.bindIf);
return show ? value: '';
}
ngModel.$parsers.push(parser);
}
};
});
HTML:
<input type="text" bind-if="displayNameData[0].show"
ng-model="displayNameData[0].value">

Answer 3

You can bind ng-model to function
Binding to a getter/setter
Sometimes it's helpful to bind ngModel to a
getter/setter function. A getter/setter is a function that returns a
representation of the model when called with zero arguments, and sets
the internal state of a model when called with an argument. It's
sometimes useful to use this for models that have an internal
representation that's different from what the model exposes to the
view.
index.html
<div ng-controller="ExampleController">
<form name="userForm">
<label>Name:
<input type="text" name="userName"
ng-model="user.name"
ng-model-options="{ getterSetter: true }" />
</label>
</form>
<pre>user.name = <span ng-bind="user.name()"></span></pre>
</div>
app.js
angular.module('getterSetterExample', [])
.controller('ExampleController', ['$scope', function($scope) {
var _name = 'Brian';
$scope.user = {
name: function(newName) {
// Note that newName can be undefined for two reasons:
// 1. Because it is called as a getter and thus called with no arguments
// 2. Because the property should actually be set to undefined. This happens e.g. if the
// input is invalid
return arguments.length ? (_name = newName) : _name;
}
};
}]);