Inline Error Messages with Rails 3

I was recently tasked with converting a Rails app from Rails 2.3.x to 3.2.x.

“Shouldn’t be too difficult” I thought, but one of the first things that came out and bit me is that the error_message_on helper method, which was previously used to display error messages next to the form fields that had caused them, has been deprecated.

It took me a little while to figure out how to reinstate this functionality. Here’s how I did it.

First off, we should probably note, that although this method has been removed from Rails, it is now available as a plugin named dynamic_form. This means that you could just add dynamic_form to your gem file, run bundler and everything would be the same as before.

However, as Ryan Bates notes in Railscast 211 (Validations in Rails 3), the reason that this and a couple of other methods (such as error_messages_for) have been removed, is that the display of error messages often needs to be customized and doing this through the old methods was a little bit cumbersome.

Instead we now have access to @resource.errors which is an instance of the class ActiveModel::Errors containing all errors for a particular resource, where each key is the attribute name and the value is an array of strings with all errors.

That means that we can now write:

<% if @resource.errors[:field_name] %>
  <span class="formError">
    <%= @resource.errors[:field_name][0] %>
  </span>
<% end %>

and have our errors reappear back inline.

Note, that it is necessary to write [:field_name][0], as @resource.errors[:field_name] is an array containing all available error messages and it is probably not a good idea to display all of these to your users in one go.

Now, this is all a bit verbose, so it is a good idea to move this code into a helper method.

def error_message_for(field, options = {:prepend_text => ""})
  error_message = @resource.errors[field][0]
  if error_message
    raw "<span class='formError'>#{options[:prepend_text]} #{error_message}</span>"
  end
end

Now you can just write:

<%= error_message_for(:field_name, :prepend_text => "Whatever ") %>

References

• Rails 3 – show error messages next to field
• Rails 3 – inline errors
• Working with Validation Errors