Support dynamic form fields by validating the selected value of a dro…

…pdown.

Issue [2723](thoughtbot#2723)

I want to create dynamic dropdowns in a form.
Update a field based on the selected value of another dropdown.
I achieve this by calling Model.new model_params in the controller, which is currently not supported

app/controllers/administrate/application_controller.rb

@@ -26,7 +26,11 @@ def show
     end
 
     def new
-      resource = new_resource
+      if params[resource_class.model_name.element].present?
+        resource = new_resource(resource_params)
+      else
+        resource = new_resource
+      end
       authorize_resource(resource)
       render locals: {
         page: Administrate::Page::Form.new(dashboard, resource)