vue.js - how to watch property of nested object and get index?

What is the best way to watch the property and get an index of nested object. Look at my code already solve it manually by passing method in select field @change="operatorChanged(key)"

But in my real project, I'm using https://vue-multiselect.js.org/ instead of normal select tag. In multiselect component I didn't get way to pass key in method. They only provide @select="method" but not allow parameter.

JSFiddle link

Here is my template HTML.

<div class="flexi-area">
<div class="flexi-number-list">
    <div class="flexi-numbers" v-for="(flex, key) in mt.flexi">  

        <input type="number" placeholder="Number" autocomplete="off" id="number" v-model="flex.number" v-on:keyup="numberChange(key)" required>

        <!-- this is created by me-->
        <select v-model="flex.operator_id" @change="operatorChanged(key)">
            <option value="">Operator</option>
            <option v-for="operator in operatorList" :value="operator"> 
                {{ operator.name }}
            </option> 
        </select>

        <!-- this is multiselect field -->
        <multiselect :allow-empty="false" deselect-label="" select-label="" v-model="flex.operator_id" :options="operatorList" :preserve-search="true" placeholder="Operator" label="name" track-by="name" :preselect-first="false" @onChange="operatorChanged(key)">
            <template slot="tag" slot-scope="props">
                <span>{{ props.option.name }}</span>
                <span @click="props.remove(props.option)">x</span> 
            </template>
        </multiselect>  

        <select v-model="flex.type"> 
            <option v-if="!flex.operator_id" value="">Type</option>

            <template v-if="flex.operator_id">  
                <template v-if="flex.operator_id.type == 'flexiload'">  
                    <option value="prepaid">Prepaid</option>
                    <option value="postpaid">Postpaid</option> 
                </template>

                <template v-else-if="flex.operator_id.type == 'mobile-banking'"> 
                    <option value="personal">Personal</option>
                    <option value="agent">Agent</option>
                </template> 
            </template>   
        </select>

        <input type="number" autocomplete="off"  placeholder="Amount" v-on:keyup="amountChange(key)" id="amount" v-model="flex.amount" required > 
    </div><!--  /.flexi-numbers -->
</div> <!-- /.flexi-number-list --> 
<input type="password" placeholder="Pin" id="pin" v-model="mt.pin" required></div>

and here is my vue js code.

export default {  
data: function () {
    return {
        mt: {  
            flexi: [ 
                { number: '', operator_id: '', type: '', amount: '' }, 
                { number: '', operator_id: '', type: '', amount: '' }, 
            ],
            pin: '',
        }, 
        operatorList: [
            { id: 1, name: 'Grameenphone', type: 'flexiload' },
            { id: 2, name: 'Banglalink', type: 'flexiload' },
            { id: 3, name: 'Bkash', type: 'mobile-banking' },
            { id: 4, name: 'Rocket', type: 'mobile-banking' },
        ],
    }
},
watch: {  
    // is any way to watch operator_id value or object index like follwing?
    /*
    'mt.flexi.operator_id': function (index, value) {

    }
    */
}, 
methods: {  
    numberChange(key) { 
      /* Here I can get the number */
      //this.mt.flexi[key].number;          
    },
    amountChange(key) { 
        alert(key);
      /* Here I can get the amount */
      //this.mt.flexi[key].amount;          
    },
    operatorChanged( key ) {
        alert(key);
      if ( this.mt.flexi[key].operator_id ) {
        if ( this.mt.flexi[key].operator_id.type == 'flexiload' ) {
          this.mt.flexi[key].type = 'prepaid'; 
        } else if ( this.mt.flexi[key].operator_id.type == 'mobile-banking' ) {
          this.mt.flexi[key].type = 'personal';
        }  
      }   
      //this.amountChange(key);
    },
} //methods } //export default