angularJS-select级联

<div ng-init="districts=[
{code: '99', name: '省直', units: [
{code: '9901', name: '省单位1', positions: [
{code: '990101', name: '省职位1' },
{code: '990102', name: '省职位2' }
]},
{code: '9902', name: '省单位2', positions: [
{code: '990201', name: '省职位3' },
{code: '990202', name: '省职位4' }
]}
]},
{code: '01', name: '长沙', units: [
{code: '0101', name: '长沙单位1', positions: [
{code: '010101', name: '长沙职位1' },
{code: '010102', name: '长沙职位2' }
]},
{code: '0201', name: '长沙单位2', positions: [
{code: '010201', name: '长沙职位3' },
{code: '010202', name: '长沙职位4' }
]}
]}]" >
<select ng-model="selectedDistrict" ng-change="selectedUnit=''" ng-options="district.name for district in districts" >
<option value="">---请选择考区---</option>
</select>
<select ng-model="selectedUnit" ng-change="selectedPosition=''" ng-options="unit.name for unit in selectedDistrict.units">
<option value="">---请选择单位---</option>
</select>
<select ng-model="selectedPosition" ng-options="unit.name for unit in selectedUnit.positions">
<option value="">---请选择职位---</option>
</select>
</div>