disable or indicate the default sorting in angular smart-table

Question

angular 1 there are 3 sort options: asc, default and desc how can I change the sort so the sort will be only between desc and asc and will show the default also only on the loading as happened in every table; the situation now is that there is situation with no sort at all in the third click;

thx example :

<form>
    <label for="predicate">selected predicate:</label>
    <select class="form-control" id="predicate" ng-model="selectedPredicate" ng-options="predicate for predicate in predicates"></select>
</form>
<table st-table="rowCollection" class="table table-striped">
    <thead>
    <tr>
        <th st-sort="firstName">first name</th>
        <th st-sort="lastName">last name</th>
        <th st-sort="birthDate">birth date</th>
        <th st-sort="balance">balance</th>
        <th>email</th>
    </tr>
    <tr>
        <th>
            <input st-search="firstName" placeholder="search for firstname" class="input-sm form-control" type="search"/>
        </th>
        <th colspan="3">
            <input st-search placeholder="global search" class="input-sm form-control" type="search"/>
        </th>
        <th>
            <input st-search="{{selectedPredicate}}" placeholder="bound predicate" class="input-sm form-control" type="search"/>
        </th>
    </tr>
    </thead>
    <tbody>
    <tr ng-repeat="row in rowCollection">
        <td>{{row.firstName | uppercase}}</td>
        <td>{{row.lastName}}</td>
        <td>{{row.birthDate | date}}</td>
        <td>{{row.balance | currency}}</td>
        <td><a ng-href="mailto:{{row.email}}">email</a></td>
    </tr>
    </tbody>
</table>

js:

app.controller('filterCtrl', ['$scope', '$filter', function (scope, filter) {
    scope.rowCollection = [
        {firstName: 'Laurent', lastName: 'Renard', birthDate: new Date('1987-05-21'), balance: 102, email: '[email protected]'},
        {firstName: 'Blandine', lastName: 'Faivre', birthDate: new Date('1987-04-25'), balance: -2323.22, email: '[email protected]'},
        {firstName: 'Francoise', lastName: 'Frere', birthDate: new Date('1955-08-27'), balance: 42343, email: '[email protected]'}
    ];

    scope.predicates = ['firstName', 'lastName', 'birthDate', 'balance', 'email'];
    scope.selectedPredicate = scope.predicates[0];
}]);

Answer 1

Found the solution:

adding st-skip-natural="true" for each field

<th class="tableColumn" st-skip-natural="true" st-sort>date</th>