title: Validation rule for applying to single and default scenario
body: <p>I'm using Yii2 validator to validate my attributes. I have an validator rule, which should be fire on an specific scenario by using <code>$model-&gt;validate('run_special_validation');</code> and also at the default scenario when running <code>$model-&gt;validate();</code></p>
  
  <p>I know, that I can define the scenarios like this:</p>
  
  <pre><code>public function rules() {
      return [ 
          ['name', 'required', 'on' =&gt; 'run_special_validation'],
          ['email', 'required'],
      ];
  }
  </code></pre>
  
  <p>What I need is to run the validation for <code>name</code> separately, for example by using <code>$model-&gt;validate('run_special_validation')</code>  and also together with all other validations by running <code>$model-&gt;validate()</code>.</p>
  
  <p>//Edit: I know, there is an "ad hoc" validation available in Yii2, which maybe could be used. (<a href="https://www.yiiframework.com/doc/guide/2.0/en/input-validation#ad-hoc-validation" rel="nofollow noreferrer">https://www.yiiframework.com/doc/guide/2.0/en/input-validation#ad-hoc-validation</a>)  But is there a way to do that insight the model scope?</p>