title: How to set uniqueness for multiple fields in ActiveRecord (Yii2)?
body: <p>How to set uniqueness for multiple fields in ActiveRecord (Yii2)?
  I have tried as written in manual</p>
  
  <pre><code>['a1', 'unique', 'targetAttribute' =&gt; ['a1', 'a2']]
  </code></pre>
  
  <p>But it doesn't work.</p>