Polymorphic Associations and Validation

Hi!

I want to validate values from a form. That wouldn't be very difficult, but the model I want to polulate is in a polymorphic association! Here are my models:

* Document (created_at, autor, resource_id, resource_type)    ** Folder (name)    ** Binary (name, data, content_type)

So a document can be either a folder or a binary. But here the problem begins. Folder and Binary both have a "name"-column (this column is not in the "documents"-table), so when users create a folder, they can enter a name. When they upload a file, the "name"-column of the "binaries"-table will be set automatically to the name of the file the user has uploaded. That's no problem! But I want to validate the name for a folder the user enteres. I have two seperate forms for folder- and binary-creation, and I simply want to validate the presence and the length of a new folder-name. But where does the validation go? And how can I display the error- messages? I tried several things, but when I try to enter a folder without a name there will be no new record in "folders" (which is good!) but no error-message and an entry in the "documents"-table. How can I validate those entries and how can I display error-messages? And how can I prevent inserting "empty" entries to the "documents"- table?

Thanks for any help, - Dominik