Partial-Form validation from multiple pages (f.e. login-partial)

Hello - hard question:

I have my login-partial in my application.html.erb-layout.

Now I want to validate it, if the user pushes the submit button and show error-messages for it, if e.g. the passwordfield is empty.

But how do I do that? For getting the error messages, I must use

render :action => 'create'

But there I have to know, from which action I came from. Beside different pages, need different seperate instance variables (which arn't reproduced with render :action).

Second try with

redirect_to :back

looses the errors-array...!

How can I solve this problem? Found nothing in the net =( Beside: I'm using restful_authentication-plugin

HEEELP!