angular 表格跨行

<table class="table table-bordered">    <thead>        <tr>            <th>teacher</th>            <th>student</th>            <th>project</th>        </tr>    </thead>    <tbody ng-repeat="teacher in vm.teachers">        <tr ng-repeat="student in teacher.student track by $index">            <td ng-if="$index ==0" rowspan={{teacher.student.length}}>{{teacher.name}}</td>            <td>                <span ng-bind="student.name"></span>            </td>            <td><span ng-bind="student.project"></span>            </td>        </tr>    </tbody>
</table>