How to properly handle changed attributes in Rails before_save hook?

Question

I have a model that looks like this:

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 

This works fine, as device tests show:

 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 

But the offset from the @changed_attributes instance @changed_attributes simply wrong. Is there a standard Rails-y way to get the old attribute value that changes to persist?

Update: Thanks to Douglas F. Shearer and Simone Carletti (who apparently prefers Murphy Guinness), I have a cleaner solution:

  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 

Thanks guys!

Answer 1

You want ActiveModel::Dirty .

Examples:

 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'] 

Full documentation: http://api.rubyonrails.org/classes/ActiveModel/Dirty.html

Answer 2

You are using the correct function, but the wrong API. Do you need #changes and #changed? .

See this article and the official official API .

Two additional notes about your code:

• Never throw an Exception directly when you really want to save runtime errors. This is a Java style. You must save StandardError because lower errors are usually a compilation error or a system error.

• In this case, you do not need a start block.

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