如何在角度中同时使用多个字段进行排序？例如，先按组，然后按子组

$scope.divisions = [{'group':1,'sub':1}, {'group':2,'sub':10}, {'group':1,'sub':2},{'group':1,'sub':20},{'group':2,'sub':1},
    {'group':2,'sub':11}];

我想将其显示为

组：子组

1 - 1

1 - 2

1 - 20

2 - 1

2 - 10

2 - 11

<select ng-model="divs" ng-options="(d.group+' - '+d.sub) for d in divisions | orderBy:'group' | orderBy:'sub'" />