AngularJS 1.5 ngOptions Comparison for Displaying a Plain Array

Didn't find a solution using track by, but the AngularJS docs for Select had a solution using a parser and formatter so I could get away from using the hack in the question. I tweaked it a bit so if the key was a string it will leave it alone, otherwise it converts it, and this seems to work. Any criticisms or issues that I don't see please comment, otherwise hope this helps someone.

(function () {

    'use strict';

    /**
     * Binds a select field to a non-string value via ngModel parsing and formatting,
     * which simply uses these pipelines to convert the string value.
     * @constructor
     * @ngInject
     * ---
     * NOTE: In general matches between a model and an option is evaluated by strict
     * comparison of the model value against the value of the available options.
     * Setting the option value with the option's "value" attribute the value
     * will always be a "string", which means that the model value must also
     * be a string, otherwise the select directive cannot match them
     * reliably.
     */
    function selectConvertKey(_) {

        return {
            require: 'ngModel',
            link: function ($scope, $element, $attrs, $ctrl) {

                var ngModelCtrl = $ctrl;

                // Do nothing if no ng-model
                if (!ngModelCtrl) {
                    return;
                }

                // ---
                // PRIVATE METHODS.
                // ---

                /**
                 * Convert the key to a number if the key is a number.
                 * @param key
                 * @returns {Number}
                 * ---
                 * NOTE: Using Number() instead of parseInt() means that a string
                 * composed of letters and numbers, and start with a number will
                 * not be converted.
                 */
                function selectConvertKeyParser(key) {

                    var keyAsNumber = Number(key);

                    // Check if the key is not a number
                    if(_.isNaN(keyAsNumber)) {
                        return key;
                    }

                    return keyAsNumber;
                }

                /**
                 * Convert the key to a string.
                 * @param key
                 * @returns {string}
                 */
                function selectConvertKeyFormatter(key) {
                    return '' + key;
                }

                // ---
                // MODEL PROPERTIES.
                // ---

                /**
                 * Formatters used to control how the model changes are formatted
                 * in the view, also known as model-to-view conversion.
                 * ---
                 * NOTE: Formatters are not invoked when the model is changed
                 * in the view. They are only triggered if the model changes
                 * in code. So you could type forever into the input, and
                 * the formatter would never be invoked.
                 */
                ngModelCtrl.$formatters.push(selectConvertKeyFormatter);

                /**
                 * Parsers used to control how the view changes read from the
                 * DOM are sanitized/formatted prior to saving them to the
                 * model, and updating the view if required.
                 */
                ngModelCtrl.$parsers.push(selectConvertKeyParser);
            }
        };
    }

    selectConvertKey.$inject = [
        '_'
    ];

    angular
        .module('app')
        .directive('selectConvertKey', selectConvertKey);

})();