Question

我的模型看起来像这样：

class StopWord < ActiveRecord::Base
  UPDATE_KEYWORDS_BATCH_SIZE = 1000

  before_save :update_keywords

  def update_keywords
    offset = 0
    max_id = ((max_kw = Keyword.first(:order => 'id DESC')) and max_kw.id) || 0
    while offset <= max_id
      begin
        conditions = ['id >= ? AND id < ? AND language = ? AND keyword RLIKE ?',
            offset, offset + UPDATE_KEYWORDS_BATCH_SIZE, language]

        # Clear keywords that matched the old stop word
        if @changed_attributes and (old_stop_word = @changed_attributes['stop_word']) and not @new_record
          Keyword.update_all 'stopword = 0', conditions + [old_stop_word]
        end

        Keyword.update_all 'stopword = 1', conditions + [stop_word]

      rescue Exception => e
        logger.error "Skipping batch of #{UPDATE_KEYWORDS_BATCH_SIZE} keywords at offset #{offset}"
        logger.error "#{e.message}: #{e.backtrace.join "\n    "}"

      ensure
        offset += UPDATE_KEYWORDS_BATCH_SIZE
      end
    end
  end
end

这个工作正常，因为单元测试显示：

class KeywordStopWordTest < ActiveSupport::TestCase
  def test_stop_word_applied_on_create
    kw = Factory.create :keyword, :keyword => 'foo bar baz', :language => 'en'
    assert !kw.stopword, 'keyword is not a stop word by default'

    sw = Factory.create :stop_word, :stop_word => kw.keyword.split(' ')[1], :language => kw.language
    kw.reload
    assert kw.stopword, 'keyword is a stop word'
  end

  def test_stop_word_applied_on_save
    kw = Factory.create :keyword, :keyword => 'foo bar baz', :language => 'en', :stopword => true
    sw = Factory.create :keyword_stop_word, :stop_word => kw.keyword.split(' ')[1], :language => kw.language

    sw.stop_word = 'blah'
    sw.save

    kw.reload
    assert !kw.stopword, 'keyword is not a stop word'
  end
end

但是对@changed_attributes实例变量进行修改只是感觉不对。是否有标准的Rails-y方法来获取正在保存的属性的旧值？

更新：感谢Douglas F Shearer和Simone Carletti（显然更喜欢墨菲的吉尼斯），我有一个更清洁的解决方案：

  def update_keywords
    offset = 0
    max_id = ((max_kw = Keyword.first(:order => 'id DESC')) and max_kw.id) || 0
    while offset <= max_id
      begin
        conditions = ['id >= ? AND id < ? AND language = ? AND keyword RLIKE ?',
            offset, offset + UPDATE_KEYWORDS_BATCH_SIZE, language]

        # Clear keywords that matched the old stop word
        if stop_word_changed? and not @new_record
          Keyword.update_all 'stopword = 0', conditions + [stop_word_was]
        end

        Keyword.update_all 'stopword = 1', conditions + [stop_word]

      rescue StandardError => e
        logger.error "Skipping batch of #{UPDATE_KEYWORDS_BATCH_SIZE} keywords at offset #{offset}"
        logger.error "#{e.message}: #{e.backtrace.join "\n    "}"

      ensure
        offset += UPDATE_KEYWORDS_BATCH_SIZE
      end
    end
  end

谢谢，伙计们！

Answer 1

您想要ActiveModel::Dirty。

示例：

person = Person.find_by_name('Uncle Bob')
person.changed?       # => false

person.name = 'Bob'
person.changed?       # => true
person.name_changed?  # => true
person.name_was       # => 'Uncle Bob'
person.name_change    # => ['Uncle Bob', 'Bob']

完整文档：http://api.rubyonrails.org/classes/ActiveModel/Dirty.html

Answer 2

您正在使用正确的功能但错误的API。您应该#changes和#changed?。

请参阅this article和official API。

关于您的代码的另外两个注释：

当您真正想要挽救执行错误时，不要直接拯救Exception。这是Java风格。您应该抢救StandardError，因为较低的错误通常是编译错误或系统错误。

在这种情况下，您不需要开始块。

def update_keywords
  ...
  rescue => e
  ...
  ensure
  ...
end

如何在Rails before_save钩子中正确处理更改的属性？

2 个答案: