*   Added `undo_changes` method to `ActiveModel::Dirty` API to restore all the
    changed values to the previous data.

    *Igor G.*

*   Allow proc and symbol as values for `only_integer` of `NumericalityValidator`

    *Robin Mehner*

*   `has_secure_password` now verifies that the given password is less than 72
    characters if validations are enabled.

    Fixes #14591.

    *Akshay Vishnoi*

*   Remove deprecated `Validator#setup` without replacement.

    See #10716.

    *Kuldeep Aggarwal*

*   Add plural and singular form for length validator's default messages.

    *Abd ar-Rahman Hamid*

*   Introduce `validate` as an alias for `valid?`.

    This is more intuitive when you want to run validations but don't care about
    the return value.

    *Henrik Nyh*

Please check [4-1-stable](https://github.com/rails/rails/blob/4-1-stable/activemodel/CHANGELOG.md) for previous changes.