Proper redirect/render after a validation error occurs with adding or editing an item that’s dependent on another

Pass the ID of the model above what you’re validating (Publisher if you’re editing Title) in the URL (GET). Like so:

<%= start_form_tag :action => 'create_title', :id => @publisher.id %>

On the else for if the item doesn’t save (doesn’t pass validation), just put:

redirect_to :action => 'name_of_the_action_where_you_started'

Using redirect_to will mean that you don’t have to specify the ID of the model above what’s being validated, because it doesn’t redirect the whole page–just the part of the view that corresponds to that action. (is that right?)