gistfile1.rb

class FieldError
  def self.field_error_proc
    Proc.new do |html_tag, error_data|
      render_field_with_error html_tag, error_data
    end
  end

  def self.render_field_with_error(html_tag, error_data)
    if html_tag =~ /^<label/
      return wrap_label_tag_with_label_for_field_with_errors_span(html_tag)
    end

    model = error_data.object

    model_name = error_data.object_name
    attribute_name = error_data.method_name
    human_attribute_name = model.class.human_attribute_name(attribute_name)
    attribute_errors = model.errors.on(attribute_name.to_sym)

    field_with_errors_span = wrap_html_tag_with_field_with_errors_span(html_tag)
    model_error_span = model_error_span(model_name, attribute_name, human_attribute_name, attribute_errors)
    "#{field_with_errors_span} #{model_error_span}"
  end
  
  def self.model_error_span(model_name, attribute_name, human_attribute_name, attribute_errors)
    error_message_id = model_data_error_message_id(model_name, attribute_name)    
    error_message = error_message(attribute_name, human_attribute_name, attribute_errors)
    wrap_error_message_with_field_with_errors_message_span(error_message, error_message_id)
  end
  
  def self.wrap_html_tag_with_field_with_errors_span(html_tag)
    "<span class=\"field_with_errors\">#{html_tag}</span>"
  end

  def self.wrap_label_tag_with_label_for_field_with_errors_span(span_tag)
    "<span class=\"label_for_field_with_errors\">#{span_tag}</span>"
  end

  def self.model_data_error_message_id(model_name, attribute_name)
    "#{model_name}_#{attribute_name}_error_message"
  end

  def self.wrap_error_message_with_field_with_errors_message_span(error_message, html_id)
    "<span id=\"#{html_id}\" class=\"field_with_errors_message\">#{error_message}</span>"
  end
  
  def self.error_message(attribute_name, human_attribute_name, attribute_errors)
    joined_error_messages = attribute_errors.join(", ") if attribute_errors.is_a? Array
    "#{human_attribute_name} #{(joined_error_messages || attribute_errors)}"
  end
end