What Rails does with STI classes when using Polymorphic associations

poly_sti.rb

Loading development environment (Rails 3.2.2)
1.9.3p0 :001 > car = Car.new()
 => #<Car id: nil, name: nil, borrowable_id: nil, borrowable_type: nil, created_at: nil, updated_at: nil> 
1.9.3p0 :002 > student = Student.new
 => #<Student id: nil, name: nil, type: "Student", created_at: nil, updated_at: nil> 
1.9.3p0 :003 > student.save
   (0.3ms)  begin transaction
  SQL (8.8ms)  INSERT INTO "staffs" ("created_at", "name", "type", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Fri, 23 Mar 2012 21:37:53 UTC +00:00], ["name", nil], ["type", "Student"], ["updated_at", Fri, 23 Mar 2012 21:37:53 UTC +00:00]]
   (56.4ms)  commit transaction
 => true 
1.9.3p0 :004 > student.car = car
  Car Load (0.2ms)  SELECT "cars".* FROM "cars" WHERE "cars"."borrowable_id" = 1 AND "cars"."borrowable_type" = 'Staff' LIMIT 1
   (0.0ms)  begin transaction
  SQL (0.6ms)  INSERT INTO "cars" ("borrowable_id", "borrowable_type", "created_at", "name", "updated_at") VALUES (?, ?, ?, ?, ?)  [["borrowable_id", 1], ["borrowable_type", "Staff"], ["created_at", Fri, 23 Mar 2012 21:38:00 UTC +00:00], ["name", nil], ["updated_at", Fri, 23 Mar 2012 21:38:00 UTC +00:00]]
   (51.0ms)  commit transaction
 => #<Car id: 1, name: nil, borrowable_id: 1, borrowable_type: "Staff", created_at: "2012-03-23 21:38:00", updated_at: "2012-03-23 21:38:00"> 
1.9.3p0 :005 > car.borrowable
  Staff Load (0.3ms)  SELECT "staffs".* FROM "staffs" WHERE "staffs"."id" = 1 LIMIT 1
 => #<Student id: 1, name: nil, type: "Student", created_at: "2012-03-23 21:37:53", updated_at: "2012-03-23 21:37:53">