form_builder_error_proc.rb

# The following code lets you define a custom field_error_proc for a template.
# This can be useful for custom form builders when you don't want the default
# field_error_proc behaviour.
#
# class CustomFormBuilder < ActionView::Helpers::FormBuilder
#   def initialize(object_name, object, template, options, proc)
#     super
#     @template.custom_field_error_proc = method(:field_error_proc)
#   end
#
#   def field_error_proc(html_tag, instance)
#     # Do your own custom error wrapping in here.
#   end
# end

class ActionView::Base
  attr_accessor :custom_field_error_proc
end

class ActionView::Helpers::InstanceTag
  alias_method :error_wrapping_without_form_builder, :error_wrapping
  def error_wrapping(html_tag, has_error)
    if custom_field_error_proc = @template_object.custom_field_error_proc
      has_error ? custom_field_error_proc.call(html_tag, self) : html_tag
    else
      error_wrapping_without_form_builder(html_tag, has_error)
    end
  end
end