password_change.rb

class PasswordChange
  include ActiveModel::Model

  attr_accessor(
    :user,
    :current_password,
    :new_password,
    :new_password_confirmation,
  )

  validates_presence_of :current_password, :new_password, :new_password_confirmation
  validate :current_password_should_be_correct
  validate :new_password_and_confirmation_should_match

  def save
    if valid?
      user.password = new_password
      user.save!
    end
  end

 private

  def current_password_should_be_correct
    errors.add(:current_password, "is invalid") unless user.authenticate?(current_password)
  end

  def new_password_and_confirmation_should_match
    errors.add(:base, "New password and new password confirmation does not match") if new_password != new_password_confirmation
  end

end