rails--rails/activemodel/CHANGELOG.md

• Raise FrozenError when trying to write attributes that aren't backed by the database on an object that is frozen:

  class Animal
    include ActiveModel::Attributes
    attribute :age
  end
  
  animal = Animal.new
  animal.freeze
  animal.age = 25 # => FrozenError, "can't modify a frozen Animal"
  

  Josh Brody

• Add *_previously_was attribute methods when dirty tracking. Example:

  pirate.update(catchphrase: "Ahoy!")
  pirate.previous_changes["catchphrase"] # => ["Thar She Blows!", "Ahoy!"]
  pirate.catchphrase_previously_was # => "Thar She Blows!"
  

  DHH

Please check 6-0-stable for previous changes.