sti-perf-hax.rb

class MyBase < ActiveRecord::Base
  def self.compute_type(type_name)
    case type_name
    when "MyBase"; MyBase
    when "Foo";    Foo
    when "Bar";    Bar
    else super
    end
  end
end

class Foo < MyBase
end

class Bar < MyBase
end

Anyway, ActiveSupport::Dependencies.constantize(type_name) isn't so bad.