Edit in JSFiddle

angular.module('app', []);

function Ctrl($scope) {
    $scope.categories = [{
        '_id': 'ramen',
            'price': '3000',
            'parent': null
    }, {
        '_id': 'gimbab',
            'price': '3500',
            'parent': null
    }, {
        '_id': 'gimchi ramen',
            'price': '3500',
            'parent': 'ramen'
    }];

    $scope.filterParentNull = function (category) {
        return category.parent === null;
    };
}

<table class="table table-striped" ng-controller="Ctrl">
    <tr ng-repeat="category in categories | filter:filterParentNull">
        <td>{{$index + 1}}</td>
        <td>{{category._id}}</td>
        <td>{{category.price}}</td>
    </tr>
</table>