title: Vuejs key in v-for loop with simple array
body: <p>I want to build a dynamic form with list groups and user can add, remove and reorder them, with transition. But i dont know how to set key for each item inside v-for loop. As document say use object as key is bad idea, or use index as same as not use key at all.
  Could anyone show me a good solution for this.</p>
  
  <pre><code>&lt;div id="root"&gt;     
      &lt;transition-group name="flip-list" tag="div"&gt;
          &lt;div class="form-group" v-for="(item, index) in items" :key="what key to use"&gt;              
              &lt;input type="text" name="name" v-model="item.name"&gt;
              &lt;input type="tel" name="tel" v-model="item.tel"&gt;
          &lt;/div&gt;
      &lt;/transition-group&gt;
      &lt;button @click="add_item"&gt;Add&lt;/button&gt;      
  &lt;/div&gt;
  
  &lt;script&gt;
       new Vue({
          el: '#root',
          data: {
              items: []               
          },
          methods: {
              add_item: function () {
                  this.items.push({
                      name: '',
                      tel: ''
                  })
              }
          }           
      })
  &lt;/script&gt;
  </code></pre>