Rails: ActiveModel でバリデーションエラーを自分で起こす ( raise ActiveRecord::RecordInvalid ) ref: http://qiita.com/Yinaura/items/24997f0005e187f54210

book.rb

class Book
  include ActiveModel::Model

  attr_accessor :title

  validates :title, presence: true

  def self.new! (params = [])
    model = self.new(params)

      # invalid? は破壊的メソッド。ここで model にエラー情報が入る。
      if model.invalid?
        # エラー内容を反映した例外が起こる
        raise ActiveRecord::RecordInvalid.new(model)
      end
    model
  end
end

file1.txt

Book.raise_validation_error

file2.txt

Book.new(params)

file3.txt

Book.new.raise_validation_error

file4.txt

Book.new(params)