after_commitで変更されたカラムの情報を取得する

class User < ApplicationRecord
  after_save do
    p "after_save"
    p "changes"
    p self.changes
    p "previous_changes"
    p self.previous_changes
  end

  after_commit do
    p "after_commit"
    p "changes"
    p self.changes
    p "previous_changes"
    p self.previous_changes
  end
end

u = User.find 1
u.name = "BBB"
u.save

# "after_save"
# "changes"
# {"name"=>["AAA", "BBB"], "updated_at"=>[Tue, 09 Aug 2016 10:57:44 UTC +00:00, Tue, 09 Aug 2016 10:58:51 UTC +00:00]}
# "previous_changes"
# {}

# "after_commit"
# "changes"
# {}
# "previous_changes"
# {"name"=>["AAA", "BBB"], "updated_at"=>[Tue, 09 Aug 2016 10:57:44 UTC +00:00, Tue, 09 Aug 2016 10:58:51 UTC +00:00]}