Angularjs ngOptions-在表达式中使用track by时，筛选器不起作用

请看这本书：

为什么筛选器不使用在表达式中具有“跟踪依据”的ng选项，而使用“选择为”时该选项也不起作用

<span> state selector : </span>
<select ng-model="filter.stateID" 
      ng-options="item.stateID as item.state for item in st_option"></select>
<br>
<span>county selector: select as</span>
<select  
      ng-model="filter.countyID" 
      ng-options="item.countyID as item.county for item in co_option | filter:{ co_state_id : filter.stateID }">
</select>
<br>
<span>county selector: track by</span>
<select  
      ng-model="filter.countyID2" 
      ng-options="item.county for item in co_option track by item.countyID | filter:{ co_state_id : filter.stateID }">
</select>    

状态选择器：


县选择器：选择为


县选择器：跟踪

更改

ng-options="item.county for item in co_option track by item.countyID | filter:{ co_state_id : filter.stateID }">

到

更改为：

<span>county selector: track by</span>
  <select ng-model="filter.countyID2" 
          ng-options="item.county for item in co_option | filter:{ co_state_id : filter.stateID } track by item.countyID">
  </select>

县选择器：跟踪方式

<span>county selector: track by</span>
  <select ng-model="filter.countyID2" 
          ng-options="item.county for item in co_option | filter:{ co_state_id : filter.stateID } track by item.countyID">
  </select>