after_commit内ではchangesが空になってるので注意

require "active_record"
ActiveRecord::Base.establish_connection(:adapter => "sqlite3", :database => ":memory:")
ActiveRecord::Migration.verbose = false
ActiveRecord::Schema.define do
  create_table :users do |t|
  end
end
class User < ActiveRecord::Base
  after_save do
    changes                     # => {"id"=>[nil, 1]}
  end
  after_commit do
    changes                     # => {}
  end
end
User.create!

ちょいとハマりました。