Why polymorphic association doesn’t work for STI if type column of the polymorphic association doesn’t point to the base model of STI?

Good question. I had exactly the same problem using Rails 3.1. Looks like you can not do this, because it does not work. Probably it is an intended behavior. Apparently, using polymorphic associations in combination with Single Table Inheritance (STI) in Rails is a bit complicated.

The current Rails documentation for Rails 3.2 gives this advice for combining polymorphic associations and STI:

Using polymorphic associations in combination with single table
inheritance (STI) is a little tricky. In order for the associations to
work as expected, ensure that you store the base model for the STI
models in the type column of the polymorphic association.

In your case the base model would be “Staff”, i.e. “borrowable_type” should be “Staff” for all items, not “Guard”. It is possible to make the derived class appear as the base class by using “becomes” : guard.becomes(Staff). One could set the column “borrowable_type” directly to the base class “Staff”, or as the Rails Documentation suggests, convert it automatically using

class Car < ActiveRecord::Base
  ..
  def borrowable_type=(sType)
     super(sType.to_s.classify.constantize.base_class.to_s)
  end