title: Yii2 : Getting class name from relation attribute
body: <p>I went through all API documentation of Yii 2.0 to find a way to reverse back to relation class name from a model attribute.</p>
  
  <p>let us suppose that class Customer has a relation 
  <code>$this-&gt;hasOne(Country::className(), ['id' =&gt; 'countryId']);</code></p>
  
  <p>and in a controller function the parameter was the attribute "countryId". How is it possible to detect the class name for the related model</p>