undefined method `before_validation'が発生

解決

ActiveModel::Validations::Callbacks をincludeする

ruby - How can I use before validation callback with form object in Rails 4? - Stack Overflow

class TestForm
  include ActiveModel::Model
  ActiveModel::Validations::Callbacks

  before_validation :format_hogehoge

  protected
  def format_hoge
    if hoge.present?
      self.hoge = NKF.nkf('-w --katakana', hoge.gsub(/[\s　]/, ''))
    end
  end
end

事象

test_form = TestForm.new(params)
test_form.invalid? #<= エラー

class TestForm
  include ActiveModel::Model

  before_validation :format_hogehoge

  protected
  def format_hoge
    if hoge.present?
      self.hoge = NKF.nkf('-w --katakana', hoge.gsub(/[\s　]/, ''))
    end
  end
end