Dropdownlist Depending On Another Dropdonwlist

I have a user creation form where there is a dropdown to set if the new user will be superuser or not:

	<div class="row">

		<?php echo $form->labelEx($model,'superuser'); ?>

		<?php echo $form->dropDownList($model, 'superuser', User::itemAlias('AdminStatus')); ?>

		<?php echo $form->error($model,'superuser'); ?>


This dropdown has only two values (‘Yes’ and 'No). My form is actually working fine, but now I need that if ‘No’ is selected in this dropdown, I need another dropdown added to the form with ‘City’ list (this list will need to be loaded from DB using ajax).

So, the ‘user’ table has a field called ‘city_id’. This field would be NULL if the new user is a superuser (‘superuser’ = ‘Yes’) and it will keep the ‘city_id’ if the user is not a superuser (‘superuser’ = ‘No’)

Can someone help me?