如何在角度同时使用多个字段进行排序？例如，先按组再按子组

$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'" />