关于element-ui 下拉菜单 el-dropdown-menu 组件 不能动态渲染 数据怎么解决？

element-ui 官网中的例子是这样写的

<el-dropdown>
  <span class="el-dropdown-link">
    下拉菜单<i class="el-icon-arrow-down el-icon--right"></i>
  </span>
  <el-dropdown-menu slot="dropdown">
    <el-dropdown-item>黄金糕</el-dropdown-item>
    <el-dropdown-item>狮子头</el-dropdown-item>
    <el-dropdown-item>螺蛳粉</el-dropdown-item>
    <el-dropdown-item disabled>双皮奶</el-dropdown-item>
    <el-dropdown-item divided>蚵仔煎</el-dropdown-item>
  </el-dropdown-menu>
</el-dropdown>

<style>
  .el-dropdown-link {
    cursor: pointer;
    color: #409EFF;
  }
  .el-icon-arrow-down {
    font-size: 12px;
  }
</style>

写法一

我一开始是这样写的，循环的是整个 el-dropdown-item 组件，
v-for语句写在了el-dropdown-menu 里,发现页面上只能渲染出列表中最后一个元素。
说明这种写法不能动态渲染数据

 <template>
<div>
    <el-dropdown trigger="click" >
          <span class="el-dropdown-link" ref="ellink" >
            {{ test_obj.title }}<i class="el-icon-arrow-down el-icon--right" />
          </span>
          <el-dropdown-menu slot="dropdown" align="center" v-for="item in test_obj.arr" :key="item.id">
            <el-dropdown-item   > 
              {{ item.desc }}
            </el-dropdown-item>
          </el-dropdown-menu>
        </el-dropdown>
</div>
</template>
<script>
export default {
    name:'test',
    data(){
        return {
            test_obj:{
                title:'测试下拉菜单',
                arr:[
                    {id:1,desc:'测试1'},
                    {id:2,desc:'测试2'},
                    {id:3,desc:'测试3'}
                ]
            }
            
        }
    }
}
</script>

结果只显示列表中最后一个数据

写法二
换成以下写法，直接在 el-dropdown-item 组件里写v-for语句，发现可以动态渲染，满足我项目中的需求

 <el-dropdown trigger="click" ref="eldrop">
          <span class="el-dropdown-link" ref="ellink" >
            {{ Global.default_department.name }}<i class="el-icon-arrow-down el-icon--right" />
          </span>
  <el-dropdown-menu slot="dropdown" align="center" >
    <el-dropdown-item v-for="department in Global.departments" @click.native="getDepartment(department)" :key="department.id"> 
      {{ department.name }}
    </el-dropdown-item>
  </el-dropdown-menu>
</el-dropdown>

查看了下两种写法渲染后的真实html内容如下

写法一的，只渲染最后一组的html

    <ul class="el-dropdown-menu el-popper" align="center" id="dropdown-menu-3569" style="transform-origin: center top; z-index: 2011; position: absolute; top: 27px; left: 672px;" x-placement="bottom-end">
        <li tabindex="-1" class="el-dropdown-menu__item">
            测试1
        </li>
        <div x-arrow="" class="popper__arrow" style="left: 19px;"></div>
    </ul>
    <ul class="el-dropdown-menu el-popper" align="center" id="dropdown-menu-3569" style="transform-origin: center top; z-index: 2012; position: absolute; top: 27px; left: 672px;" x-placement="bottom-end">
        <li tabindex="-1" class="el-dropdown-menu__item"> 
                    测试2
        </li>
        <div x-arrow="" class="popper__arrow" style="left: 19px;"></div>
    </ul>
    <ul class="el-dropdown-menu el-popper" align="center" id="dropdown-menu-3569" style="transform-origin: center top; z-index: 2013; position: absolute; top: 27px; left: 672px;" x-placement="bottom-end">
        <li tabindex="-1" class="el-dropdown-menu__item">
            测试3
        </li>
        <div x-arrow="" class="popper__arrow" style="left: 19px;"></div>
    </ul>

写法二的，正常渲染了的html

    <ul class="el-dropdown-menu el-popper" align="center" id="dropdown-menu-3203" style="position: absolute; top: 27px; left: 809px;      transform-origin: center top; z-index: 2007;" x-placement="bottom-end">
        <li tabindex="-1" class="el-dropdown-menu__item">
            测试A
        </li>
        <li tabindex="-1" class="el-dropdown-menu__item">
            测试B
        </li>
        <li tabindex="-1" class="el-dropdown-menu__item">
            测试C
        </li>
        <div x-arrow="" class="popper__arrow" style="left: 8px;"></div>
    </ul>

可以得出，写法二其实也渲染了三个li元素，但是位置重合了，根据z-index大小，只显示最后一个数据。
所以写下拉菜单时，只循环el-dropdown-item组件中的内容就可以正常动态渲染数据。