title: Angular 4 - Add Class if ngModel is not empty
body: <p>How can I add a class to an element on a page if my defined ngModel for a select element is not empty.  Currently have this..</p>
  
  <pre><code>&lt;md-select placeholder="Location" name="location" [(ngModel)]="selectedLocation"&gt;
        &lt;md-option *ngFor="let option of locationOptions" [value]="option.value"&gt;
          {{option.viewValue}}
        &lt;/md-option&gt;
      &lt;/md-select&gt;
  </code></pre>
  
  <p>And would like to add a class to a span element if the ngModel is not empty, something like this...</p>
  
  <pre><code>&lt;p&gt;Location:&lt;span ng-class="{'complete': selectedLocation.length &gt; 0}"&gt;{{ selectedLocation }}&lt;/span&gt;&lt;/p&gt;
  </code></pre>