STI and nested model forms

# Allow the "type" field to be mass assigned. This is useful
# for, in my case, nested model forms. It IS dangerous but
# with proper form validation on this type field, it should be
# safe.
class Friend < ActiveRecord::Base
  # Basic validation of the type field
  validates_format_of inheritance_column, :with => /^Friend::([A-Z][a-zA-Z]+)$/

  # More validation for the type field to be safe
  def validate
    if errors.on(self.class.inheritance_column).empty?
      begin
        Kernel.const_get(type)
      rescue NameError
        errors.add(:type, "should not be some sneaky value.")
      end
    end
  end

  private
    # The sneaky snake trick to allow "type" magic column
    # to work with nested model forms.
    def attributes_protected_by_default
      super - [self.class.inheritance_column]
    end
end