element 表单验证 el-form-item里面验证两个select 必填，select 值绑定为对象时如何验证必填

el-form-item  里面有多个select 或者input :
　　

<el-form-item label="选项一：" required>
                        <div class="diasubject">
                            <el-form-item prop="stage.code" :rules="[{ required: true, message: '请选择', trigger: 'change' }]">
                                <el-select v-model="Newtextbooks.cinfigForm.stage" placeholder="请选择" value-key="code">
                                    <el-option v-for="item in diaFormQArr.stageArr" :key="item.code" :label="item.name" :value="item">
                                    el-option>
                                el-select>
                            el-form-item>
                            <el-form-item prop="subject.code" :rules="[{ required: true, message: '请选择', trigger: 'change' }]">
                                <el-select v-model="Newtextbooks.cinfigForm.subject" placeholder="请选择" value-key="code" disabled>
                                    <el-option v-for="item in diaFormQArr.subjectArr" :key="item.code" :label="item.name" :value="item">
                                    el-option>
                                el-select>
                            el-form-item>
                        div>
                    el-form-item>

当select 选中值绑定为对象时，el-form-item 上面的prop 只需要 监听 select 的 v-model 绑定的 stage对象里面某个属性是否有值，这样就可以验证必填