artist_controller.rb

class ArtistsController < ApplicationController

    def index
        redirect_to :action => "new"
    end


    def new
        @artist = Artist.new(params[:artist])
    end

    def create
        @artist = Artist.new(params[:artist])
        if @artist.name == "rafael"
            render :text => "ok"
        else 
           redirect_to :action => "new", :artist => @artist
       end
    end

end

new.html.erb

<% form_for @artist do |f| %>

<div>
<%= f.label :name %><br />
<%= f.text_field :name %>
</div>
<%= f.submit :Save %>

<% end %>

Line 17 should read:
render :action => 'new'