el-table下每一行都有一个el-select 选中的值都绑定到ruleForm对象下的selectedPosition数组里 这种情况下我怎样去对每一行select进行验证呢 比如说第一行的select如果没有选中 在blur的时候验证这个select有没有选中值
<el-form :model="ruleForm" :rules='rules'>

    <el-table 
      :data="detailList" 
      style="width: 100%" 
      header-row-class-name="blackTableHeader">
      <el-table-column prop="index" label="序号"  align="center">
          <template slot-scope="scope">
              {{scope.$index+1}} 
        </template>
      </el-table-column>
      <el-table-column prop="name" label="人员"  align="center"></el-table-column>
      <el-table-column prop="position" label="备勤职务" align="center">
          <template slot-scope="scope">
          <el-form-item prop="selectedPosition" :rules="[{ required: true, message: '请选择职务', trigger: 'blur'}]">
              <el-select 
                  v-model="ruleForm.selectedPosition[scope.$index]" 
                  @change="handleChange(scope.row,$event)" 
                  placeholder="请选择">
                <el-option
                  v-for="(item,idx) in positionList"
                  :key="idx"
                  :label="item.name"
                  :value="item.dataKey">
                </el-option>
             </el-select>
          </el-form-item>
        </template>
      </el-table-column>
      <el-table-column prop="operation" label="操作" align="center">
          <template slot-scope="scope">
          <span class="delete" @click="deleteRow(scope,detailList)">删除</span>
        </template>
      </el-table-column>
    </el-table>
</el-form>
      
      ruleForm: {
                selectedPosition: []
            }