extend form builder with error messages

config/initializers/custom_form_builder.rb

@@ -11,6 +11,7 @@ class CustomFormBuilder < ActionView::Helpers::FormBuilder
     )
     t.content_tag(:div, elements.inject(&:+), class: 'form-actions')
   end
+
   def row(attribute, options={}, &block)
     t = @template
     classes = []
@@ -19,5 +20,15 @@ class CustomFormBuilder < ActionView::Helpers::FormBuilder
     classes << 'error' if object.errors[attribute].present?
     t.content_tag(:div, t.capture(&block), class: classes)
   end
+
+  def error_message(attribute, options={}, &block)
+    if errors = object.errors[attribute].presence
+      t = @template
+      classes = ['error-message']
+      classes << attribute
+      classes |= Array.wrap(options[:class])
+      t.content_tag(:span, options[:message] || errors.join(', '), class: classes)
+    end
+  end
 end
 ActionView::Base.default_form_builder = CustomFormBuilder