*   Deprecate `ActiveModel::Errors#get`, `ActiveModel::Errors#set` and
    `ActiveModel::Errors#[]=` methods that have inconsistent behaviour.

    *Wojciech Wnętrzak*

*   Change the default error message from `can't be blank` to `must exist` for
    the presence validator of the `:required` option on `belongs_to`/`has_one` associations.

    *Henrik Nygren*

*   Assigning an unknown attribute key to an `ActiveModel` instance during initialization
    will now raise `ActiveModel::AttributeAssignment::UnknownAttributeError` instead of
    `NoMethodError`

    Example:

        User.new(foo: 'some value')
        # => ActiveModel::AttributeAssignment::UnknownAttributeError: unknown attribute 'foo' for User.

    *Eugene Gilburg*

*   Extracted `ActiveRecord::AttributeAssignment` to `ActiveModel::AttributeAssignment`
    allowing to use it for any object as an includable module.

    Example:

        class Cat
          include ActiveModel::AttributeAssignment
          attr_accessor :name, :status
        end

        cat = Cat.new
        cat.assign_attributes(name: "Gorby", status: "yawning")
        cat.name # => 'Gorby'
        cat.status => 'yawning'
        cat.assign_attributes(status: "sleeping")
        cat.name # => 'Gorby'
        cat.status => 'sleeping'

    *Bogdan Gusiev*

*   Add `ActiveModel::Errors#details`

    To be able to return type of used validator, one can now call `details`
    on errors instance.

    Example:

        class User < ActiveRecord::Base
          validates :name, presence: true
        end

        user = User.new; user.valid?; user.errors.details
        => {name: [{error: :blank}]}

    *Wojciech Wnętrzak*

*   Change validates_acceptance_of to accept true by default.

    The default for validates_acceptance_of is now "1" and true.
    In the past, only "1" was the default and you were required to add
    accept: true.

*   Remove deprecated `ActiveModel::Dirty#reset_#{attribute}` and
    `ActiveModel::Dirty#reset_changes`.

    *Rafael Mendonça França*

*   Change the way in which callback chains can be halted.

    The preferred method to halt a callback chain from now on is to explicitly
    `throw(:abort)`.
    In the past, returning `false` in an ActiveModel or ActiveModel::Validations
    `before_` callback had the side effect of halting the callback chain.
    This is not recommended anymore and, depending on the value of the
    `config.active_support.halt_callback_chains_on_return_false` option, will
    either not work at all or display a deprecation warning.


Please check [4-2-stable](https://github.com/rails/rails/blob/4-2-stable/activemodel/CHANGELOG.md) for previous changes.