class Cat < ApplicationRecord
  after_save do
    p self
    p attribute_was(:age)
  end
end

irb(main):009:0> cat = Cat.first
  Cat Load (0.4ms)  SELECT "cats".* FROM "cats" ORDER BY "cats"."id" ASC LIMIT ?  [["LIMIT", 1]]
=> #<Cat id: 1, created_at: "2021-05-27 01:07:51.110557000 +0000", updated_at: "2021-05-27 01...
irb(main):010:0> cat.age=5
=> 5
irb(main):011:0> cat.save!
  TRANSACTION (0.2ms)  begin transaction
  Cat Update (0.5ms)  UPDATE "cats" SET "updated_at" = ?, "age" = ? WHERE "cats"."id" = ?  [["updated_at", "2021-05-27 01:08:29.723938"], ["age", 5], ["id", 1]]
#<Cat id: 1, created_at: "2021-05-27 01:07:51.110557000 +0000", updated_at: "2021-05-27 01:08:29.723938000 +0000", age: 5>
5
  TRANSACTION (4.7ms)  commit transaction
=> true

class Cat
  include Mongoid::Document

  field :age, type: Integer

  after_save do
    p self
    p attribute_was(:age)
  end
end

irb(main):001:0> a=Cat.create!
#<Cat _id: 60aef1652c97a617438dc9bb, age: nil>
nil
=> #<Cat _id: 60aef1652c97a617438dc9bb, age: nil>
irb(main):002:0> a.age=2
=> 2
irb(main):003:0> a.save!
#<Cat _id: 60aef1652c97a617438dc9bb, age: 2>
nil
=> true