Get Bootstrap and Rails error messages to play nicely together.

_form.html.erb

<div class="control-group <%= control_group_error(@model, :field_name) %>">
  <%= f.label :field_name, :class => "control-label" %>
  <div class="controls">
    <%= f.text_field :field_name %>
    <%= error_help_text(@model, :field_name) %>
  </div>
</div>

application.rb

# Prevent default Rails error mark up.
ActionView::Base.field_error_proc = Proc.new {|html, instance| html }

form_helper.rb

module FormHelper

  def control_group_error(model, field_name)
    "error" unless model.errors[field_name].blank?
  end

  def error_help_text(model, field_name)
    "<span class='help-inline'>#{model.errors[field_name].join(", ")}</span>".html_safe unless model.errors[field_name].blank?
  end
end