rails--rails/activemodel/CHANGELOG.md

• 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.