angular 中 select設(shè)置默認(rèn)值

一直有點(diǎn)迷糊angular中的select的使用，給select設(shè)置默認(rèn)值，有的時(shí)候是好的，有的時(shí)候則不行，
具體代碼如下，希望誰(shuí)能給我分析講解一下

<select class="input-sm form-control w-sm inline v-middle" ng-model="questionModObj.listItem.questionTypeKey">
    <option ng-repeat='item in questionModObj.typeList' value="{{item.questionTypeKey}}">{{item.questionTypeName}}</option>
</select>

$scope.questionModObj = {
    listItem:{
      questionId:$state.params.id,
      questionTypeKey:"2",
      questionDesc:"",
      questionHtml:''
    },
    typeList: [{ 
      questionTypeKey: "0", questionTypeName: "全部" 
    }, { 
      questionTypeKey: "1", questionTypeName: "購(gòu)物相關(guān)" 
    }, { 
      questionTypeKey: "2", questionTypeName: "訂單相關(guān)" 
    }, { 
      questionTypeKey: "3", questionTypeName: "售后相關(guān)" 
    }, { 
      questionTypeKey: "4", questionTypeName: "定制相關(guān)" 
    }, { 
      questionTypeKey: "5", questionTypeName: "其他" 
    }],
    init(){
      this.getData();
    },
    getData() {
    },
    publish(){
      console.log(this.listItem)
    }
  }