error_message_for not working

hi all,

I try to get error_message_for to get to work on my dorm. No luck...

For some reason the view doesn't show me the error message when I for
example leave out the first_name

Some has a clue what I'm doing wrong?

Thanks,
Stijn

I use a helperform to create a new user:
<h2>New user</h2>
</form>
<%= error_messages_for 'user' %>
<%= form_tag new_user_path %>
    <label for="user_first_name">first_name: </label><%=
text_field('user', 'first_name') %><br/>
    <label for="user_last_name">last_name: </label><%=
text_field('user', 'last_name') %><br/>
    <label for="user_email">email: </label><%= text_field('user',
'email') %><br/>
    <input type="submit" value="new">
<%= end_form_tag %>

the controller
  def new
    @user = User.new(params[:user])
    if @user.save
      flash[:notice] = "Nieuwe gebruiker aangemaakt"
    end
    redirect_to :back
  end

the model
class User < ActiveRecord::Base
  has_and_belongs_to_many :nodes
  has_many :posts
  validates_presence_of :first_name, :last_name
end

Apparently the problem is caused by the redirect_to :back . I need to
use render to preserve the errors.

The problem is that I call the new method in my partial, used in
different views. I thus need to redirect back. Can I use render to
send me back to the previous page?
Thanks,
Stijn

yep, i want to know that too.
how use error_message_for on redirect_to action ?
who's know that?
i know use flash only:

  def new
    @user = User.new(params[:user])
    if @user.save
      flash[:notice] = "Nieuwe gebruiker aangemaakt"
    else
      flash[:error] = @user.errors.each {|field, msg| error << msg +
'<br>' }
    end
    redirect_to :back
  end