form_builder_extensions.rb

# Chuck this in config/initializers for view-first validations
ActionView::Base.default_form_builder.class_eval do
  def error(field)
    errors(field).first
  end
  def errors(field)
    object.errors.for(field)
  end
end

ActiveRecord::Errors.class_eval do
  def generate_message(attribute, message = :invalid, options = {})
    message
  end

  def for(attribute)
    case err = self.on(attribute)
    when String, Symbol: [err]
    when Array: err
    when NilClass: []
    end
  end
end

register.haml

-# And you can use it in a view like so:

- form_for :user, @user, register_path do |f|
  %fieldset
    != f.label :name
    != f.text_field :name
    - if f.error(:name) === :blank
      .error.name_blank= _("You don't have a name??")
      
    != f.label :email
    != f.text_field :email
    - case f.error(:email)
    - when :blank
      .error.email_blank= _("You don't have an email??")
    - when :invalid
      .error.email_format= _("That doesn't look like an email address!")
    - when :taken
      .error.email_taken
        _("There is an account already registered with that email address.")
        %a{:href => root_path(:email => @user.email)} _("Log in instead?")
    - when :foo
      .error.foo= _("You need to foo first")
      

user.rb

class User < ActiveRecord::Base
  validates_presence_of :email, :name, :password,
  validates_format_of :email, :with => /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i, :allow_blank => true
  validates_uniqueness_of :email, :case_sensitive => false
  my_custom_validates_of :email, :message => :foo
end