The default error messages can be a bit stale and off putting. Try somethings like this:

error_messages_for( :user, :header_message => "Oops - We couldn't save your user!", :message => "The following fields were a bit of a problem:", :header_tag => :h1 )

You can also use error_messages_for as follows <% form_for User.new do |f| %> <%= f.error_messages :header_message => "..." %> <% end %>

By default fields that are invalid are wrapped in:

<div class="fieldWithErrors">
<input type="text" name="blah">
</div>

To override and wrap in spans instead of divs place the following in your environment.rb:

ActionView::Base.field_error_proc = Proc.new { |html_tag, instance| "<span class=\\"fieldWithErrors\\">#{html_tag}</span>" }

or to not use wrapping at all:

ActionView::Base.field_error_proc = Proc.new { |html_tag, instance| "#{html_tag}" }

By default, the error messages translate the names of the attributes through String#humanize. The way to to change that is to override the ActiveRecord::Base.human_attribute_name method.

For example, if you want to name a column in your database as :www_url and you want to say "Website" instead of "Www url" in the error message, you can put this into your model:

class Person < ActiveRecord::Base
def self.human_attribute_name(attribute_key_name)
  if attribute_key_name.to_sym == :www_url
    "Website"
  else
    super
  end
end
end

Currently this seems to be the cleanest and easiest way. Unfortunately, human_attribute_name is deprecated and may stop working in a future release of Rails.

Detailed messages for a nested model

<%@address = @order.address%>
<%=error_messages_for :address%>

The code posted by @hosiawak will still work in recent versions of Rails, but maybe a more current, idiomatic way to do it is to stick this inside the Rails::Initializer block in environment.rb (obviously you'll also need to restart your server to pick up the config change):

config.action_view.field_error_proc = Proc.new {|html_tag, instance| 
%(<span class="fieldWithErrors">#{html_tag}</span>)}

You can use Module.alias_attribute to achieve the same result as overriding human_attribute_name. After aliasing use the new name in the validates_xxxx_of methods or ActiveRecord::Errors.add.

I like to {use Hpricot}[http://www.thewebfellas.com/blog/2008/4/21/error-fields-with-a-hpricot-twist] to add error information to my form fields. Here's an example:

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
if html_tag =~ /<(input|label|textarea|select)/
  error_class = 'error'
  nodes = Hpricot(html_tag)
  nodes.each_child { |node| node[:class] = node.classes.push(error_class).join(' ') unless !node.elem? || node[:type] == 'hidden' || node.classes.include?(error_class) }
  nodes.to_html
else
  html_tag
end
end

This will only apply the CSS class 'error' to elements that aren't hidden inputs and don't already have the error class.

Sample output:

<div>
<label class="error" for="user_email">Email</label>
<input name="user[email]" size="30" class="error" type="text" id="user_email" value="" />
</div>

=== <%= error_messages_for :order, :header_message => nil, :message => nil %>

Browser view code

<div id="errorExplanation" class="errorExplanation">
<ul>
  <li>Weight 只有 1000.0</li>
  <li>Volume 只有 10.0</li>
</ul>
</div>