If there is a validation error, save action will redirect to the last known good location and set error message in a flash

app/controllers/application.rb

@@ -7,6 +7,8 @@ class ApplicationController < ActionController::Base
   # implements Instiki's legacy URLs
   require 'url_rewriting_hack'
 
+  after_filter :remember_location
+
   # For injecting a different wiki model implementation. Intended for use in tests
   def self.wiki=(the_wiki)
     # a global variable is used here because Rails reloads controller and model classes in the 
@@ -15,13 +17,37 @@ class ApplicationController < ActionController::Base
     $instiki_wiki_service = the_wiki
     logger.debug("Wiki service: #{the_wiki.to_s}")
   end
-  
+
   def self.wiki
     $instiki_wiki_service
   end
+
+  protected
   
   def wiki
     $instiki_wiki_service
   end
+
+  @@REMEMBER_NOT = []
   
+  def remember_location
+    if @response.headers['Status'] == '200 OK'
+      @session[:return_to] = url_for unless @@REMEMBER_NOT.include? action_name
+      @session[:already_tried_index_as_fallback] = false
+    end
+  end
+
+  def return_to_last_remembered
+    # Forget the redirect location
+    redirect_target, @session[:return_to] = @session[:return_to], nil
+    # then try to redirect to it
+    if redirect_target.nil?
+      raise 'Cannot redirect to index' if @session[:already_tried_index_as_fallback]
+      @session[:already_tried_index_as_fallback] = true
+      redirect_to_url '/'
+    else
+      redirect_to_url(redirect_target)
+    end
+  end
+
 end

app/controllers/wiki_controller.rb

@@ -213,21 +213,26 @@ class WikiController < ApplicationController
 
   def save
     redirect_to :action => 'index' if @page_name.nil?
-
-    if @web.pages[@page_name]
-      page = wiki.revise_page(
-          @web_name, @page_name, @params['content'], Time.now, 
-          Author.new(@params['author'], remote_ip)
-      )
-      page.unlock
-    else
-      page = wiki.write_page(
-          @web_name, @page_name, @params['content'], Time.now, 
-          Author.new(@params['author'], remote_ip)
-      )
-    end
     cookies['author'] = @params['author']
-    redirect_show(@page_name)
+
+    begin
+      if @web.pages[@page_name]
+        page = wiki.revise_page(
+            @web_name, @page_name, @params['content'], Time.now, 
+            Author.new(@params['author'], remote_ip)
+        )
+        page.unlock
+      else
+        page = wiki.write_page(
+            @web_name, @page_name, @params['content'], Time.now, 
+            Author.new(@params['author'], remote_ip)
+        )
+      end
+      redirect_show(@page_name)
+    rescue Instiki::ValidationError => e
+      flash[:error] = e
+      return_to_last_remembered
+    end
   end
 
   def show