rails delete_all exception

Question

Hello, guys!

I have some User model, Role model and Assignment model to manage role-based user system.
Just like here .

So, here some kind of code, to include roles into users form:

<% for role in Role.all %>
<%= check_box_tag "user[role_ids][]", role.id, @user.roles.include?(role) %>
<%=h role.name %><br />
<% end %>

And all's working fine for new records and for editing records.
But I just want to make possible to restrict deleting Assigment with id=1, cause this Assignment for default root user from migration, who must be admin anyway.

Otherwise, I can lose my last admin.

So I've tryed to add before_destroy to Assignment.rb model, but it doesn't work.

I've noticed, that when I'm editing user and trying to remove his admin role(assignment), the server makes a delete_all method, not destroy method.

So, does anybody know how to catch the deleting of Assignment with id=1 through this kind of code?

Thanks in advance!