title: Bind selected option $index with vue.js
body: <p>How can I bind selected option's <strong>Index</strong> using vue.js?
  Code of select:</p>
  
  <pre><code>&lt;select 
          v-model="address.districtId" 
          name="address[district_id]"
          id="address[district_id]"
          class="form-control"
      &gt;
          &lt;option value="0" selected disabled&gt;
          Choose district
          &lt;/option&gt;
          &lt;option 
              v-for="district in places[cityIndex].districts" 
              value="@{{district.id}}"
          &gt;
              @{{ district.name }}
          &lt;/option&gt;
      &lt;/select&gt;
  </code></pre>
  
  <p>I try to use </p>
  
  <pre><code>v-model:address.districtIndex="$index"
  </code></pre>
  
  <p>but It doesn't work</p>