angularjs中下拉框select 第一个为空白

<label for="userId">发起培训人</label> <select
                class="form-control  input-sm" ng-model="trainDetail.userId" name="userId" id="userId">
                <option value="{{userId}}" selected="selected">{{realName}}</option>
                <option ng-repeat="user in userList"
                    ng-selected="user.id == trainDetail.userId" value="{{user.id}}">{{user.realname}}</option>
            </select>

上述代码在页面中 如下图所示   第一行数据显示空白

 

chrome解析错误代码

<option value="? undefined:undefined ?"></option>

解决方式  ：ng-init = "trainDetail.userId = userId"  初始化  select

<label for="userId">发起培训人</label> <select
                class="form-control  input-sm" ng-model="trainDetail.userId" name="userId" id="userId" ng-init = "trainDetail.userId = userId">
                <option value="{{userId}}" selected="selected">{{realName}}</option>
                <option ng-repeat="user in userList"
                    ng-selected="user.id == trainDetail.userId" value="{{user.id}}">{{user.realname}}</option>
            </select>