irb(main):099:0>  ActiveRecord::Base.public_instance_methods.grep(/change/)
[:attachment_changes, :changed_for_autosave?, :saved_change_to_attribute, :saved_changes?, :saved_changes, :will_save_change_to_attribute?,
 :attribute_change_to_be_saved, :changes_to_save, :changed_attribute_names_to_save, :has_changes_to_save?, :saved_change_to_attribute?, :attribute_changed_in_place?,
 :changed, :clear_attribute_changes, :changes_applied, :attribute_changed?, :attribute_previously_changed?, :clear_changes_information, :changed_attributes, 
 :changed?, :previous_changes, :changes]

# 获取数据对象
irb(main):020:0> d = Notice.find(13)
# 查看原属性
irb(main):020:0> d.note
=> ""
# 修改属性，然后再保存 change, and save
irb(main):026:0> d.note='abc'
=> "abc"
irb(main):027:0> d.attribute_was :note
=> ""
irb(main):028:0> d.note
=> "abc"

irb(main):028:0> d.changed?
=> true

irb(main):029:0> d.save   
=> true
irb(main):030:0> d.note
=> "abc"

irb(main):031:0> d.attribute_was :note
=> "abc"
irb(main):032:0>  d.attribute_previously_changed? :note
=> true
irb(main):033:0> d.attribute_before_last_save :note
=> ""
irb(main):034:0>

  irb(main):034:0> d.changed?
  => false
  irb(main):034:0> d.attribute_previously_changed? :note
  => true

# 重新获取数据对象
irb(main):020:0> d = Notice.find(13)
# 查看原属性
irb(main):020:0> d.note
=> "abc"
# 直接更新数据 update directly
irb(main):020:0> d.update! note: 'abd'
   (0.2ms)  begin transaction  
   (89.1ms)  commit transaction
=> true
irb(main):023:0> d.attribute_changed? :note
=> false
irb(main):022:0> d.attribute_previously_changed? :note
=> true
irb(main):021:0> d.attribute_before_last_save :note
=> "abc"

irb(main):024:0> d.note
=> "abd"
irb(main):025:0> d.attribute_was :note
=> "abd"

# 新的ActiveRecord对象，不能追踪，说明修改信息仅仅位于内存对象中
irb(main):037:0> d2 = Notice.find(13)
  Notice Load (0.2ms)  
irb(main):039:0> d2.note
=> "abd"
irb(main):040:0> d2.attribute_previously_changed? :note
=> false
irb(main):041:0> d2.attribute_before_last_save :note
=> nil
irb(main):041:0>

# 遍历修改的属性
irb(main):044:0> (d.attribute_names - ["updated_at","created_at"]).each{|f| puts "#{f} chaned from [#{d.attribute_before_last_save(f)}] to [#{d[f]}]" if d.attribute_previously_changed?(f)}
note chaned from [abc] to [abd]

# ActiveRecord对象的方法previous_changes获取修改的属性 hash, 包含原值和新值
irb(main):079:0> d.changes
=> {}
irb(main):078:0> cs=d.previous_changes
=> {"note"=>["abc", "abd"], "updated_at"=>[Thu, 12 Nov 2020 15:18:07 CST +08:00, Thu, 12 Nov 2020 16:21:45 CST +08:00]}
# 删除无用属性
irb(main):080:0> cs.delete :updated_at
=> [Thu, 12 Nov 2020 15:18:07 CST +08:00, Thu, 12 Nov 2020 16:21:45 CST +08:00]
irb(main):081:0> cs
=> {"note"=>["abc", "abd"]}