BootstrapFormBuilder

_form.slim

= form_for resource do |f|
  = f.form_group :first_name do
    = f.label :first_name, class: 'control-label'
    = f.text_field :first_name, class: 'form-control'
    = f.error_message :first_name

application.rb

# /config/application.rb

module ProjectName
  class Application < Rails::Application
    config.action_view.default_form_builder = 'BootstrapFormBuilder'
    config.autoload_paths += %W(
      #{config.root}/lib/form_builders
    )
  end
end

bootstrap_form_builder.rb

# /lib/form_builders/bootstrap_form_builder.rb

class BootstrapFormBuilder < ActionView::Helpers::FormBuilder
  def error_message(field_name)
    return if @object.errors[field_name].blank?
    @template.content_tag :div, @object.errors[field_name].join(', '), class: 'help-block'
  end

  def form_group(field_name, options = {})
    class_def = 'form-group'
    class_def << ' has-error' unless @object.errors[field_name].blank?
    class_def << " #{options[:class]}" if options[:class].present?
    options[:class] = class_def
    @template.content_tag(:div, options) { yield }
  end
end