使用ng-options的下拉列表无法选择顶部选项

Question

我需要创建6个下拉列表来生成表格结果。我已经提取数据并填入

$scope.ScopeObject = 
    { pdts:[
        {a:" ", b:"", c:"", d:"",e:"",f:""},
        {a:" ", b:"", c:"", d:"",e:"",f:""},
        {a:" ", b:"", c:"", d:"",e:"",f:""}
    ]},
    ...

我的下拉列表1＆amp; 2将是这样的：

<select id="age" name="age" ng-model="ScopeObject.selectedAge"  class="form-control" required ng-disabled="ScopeObject.selectedAge == null" ng-options="data.age for data in ScopeObject.pdts | unique:'cover' track by data.cover">
    <option value="">-Select-</option>
</select>

<select id="selectCover" name="cover" ng-model="ScopeObject.selectedCover"  class="form-control" required ng-disabled="ScopeObject.selectedCover == null" ng-options="data.cover for data in ScopeObject.pdts  | where: {age:ScopeObject.selectedAge.age} | unique:'cover' track by data.cover">
    <option value="">-Select-</option>
</select> ` 

通过这样做，

• ddlist6 取决于 ddlist5 选择
• ddlist5＆amp; 6 取决于 ddlist4 选择
• ddlist4 取决于 ddlist3＆amp; 2＆amp; 1 选择
• ddlist3 取决于 ddlist2＆amp; 1 选择
• ddlist2 取决于+ ddlist1 +选择

这样做我无法在ddlists中创建角度第一项以自动选择第一项。有人知道这方面的解决方案吗？