example.html

<md-input-container>
    <label>Select Menu</label>
    <md-select aria-label="Select menu" ng-model="vm.model">
        <md-option ng-value="item" ng-repeat="item in vm.options track by item.id" ng-bind="item.name" select-track-by="id" select-model="vm.model"></md-option>
    </md-select>
</md-input-container>

selectTrackBy.js

'use strict';

(function () {

    angular
        .module('myApp')
        .directive('selectTrackBy', SelectTrackByDirective);

    function SelectTrackByDirective() {
        return {
            restrict: 'A',
            scope: {
                selectModel: '=',
                ngValue: '=',
                selectTrackBy: '@'
            },
            link: function (scope) {
                if (scope.selectModel && scope.ngValue[scope.selectTrackBy] === scope.selectModel[scope.selectTrackBy]) {
                    scope.selectModel = scope.ngValue;
                }
            }
        };
    }

})();