Drupal's Form API helps developers to build complex, extensible forms with minimal code and also gives option to make the form fields as dynamic. It means to change/create form fields based on the other field's given value. There is a cool trick to make dynamic fields in drupal.Normally we are using #states elements to change other field's value, show/hide based on the field's given value. I think, it is not possible to give dynamic values (for example, give values from database based on the field's given value). But we could do that with #ajax elements. Lets see the trick with an example.I want to list all students from particular selected class using a checkboxes field. If we change any class on the select box, then the current class students should be shown in the checkboxes.