Rails update behavior.

gistfile1.txt

irb(main):011:0> b.save
   (0.1ms)  begin transaction
  SQL (0.4ms)  INSERT INTO "books" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2016-03-19 00:50:25.328662"], ["updated_at", "2016-03-19 00:50:25.328662"]]
   (1.9ms)  commit transaction
=> true
irb(main):012:0> b.save
   (0.2ms)  begin transaction
   (0.1ms)  commit transaction
=> true
irb(main):013:0> b.changed?
=> false
irb(main):014:0> b.save
   (0.1ms)  begin transaction
   (0.1ms)  commit transaction
=> true