title: v-model with dynamically added property
body: <p>I have migrated an app from Vue 1 to Vue 2 and am having an issue with data from an added property on an object.</p>
  
  <p>I am looping through my data:</p>
  
  <pre><code>`&lt;template v-for="(treatments, yard) in groupByYard"&gt;`
  </code></pre>
  
  <p>and further down:</p>
  
  <pre><code>`&lt;tr v-for="treatment in treatments"&gt;`
  </code></pre>
  
  <p>groupByYard is a computed value:</p>
  
  <pre><code>`groupByYard: function() {
          var result = {};
          var treatments = this.treatments;
          for (var i = 0; i &lt; treatments.length; i++) {
              var treatment = treatments[i];
              var yard = treatment.yard.name;
              this.$set(treatment, 'completed_date', null);
              if (result[yard]) {
                  result[yard].push(treatment);
              } else {
                  result[yard] = [treatment];
              }
          }
          return result;
      }`
  </code></pre>
  
  <p>I am adding the property <strong>completed_date</strong>
      <code>this.$set(treatment, 'completed_date', null)</code></p>
  
  <p>This sets the property fine but I can't access/change this property using v-model:</p>
  
  <pre><code>`&lt;input class="form-control datepicker"
  v-model="treatment.completed_date"
  data-date-format="dd-mm-yyyy"
  data-date-end-date="0d"
  placeholder="dd-mm-yyyy"
  type="text"  /&gt;`
  </code></pre>