many-tomany creates foreign key attributes

Hello,

Use rails 2.3.x

I this a feature or a bug ?

 class Word
   has_and_belongs_to_many :sentences
 end

 class Sentence
   has_and_belongs_to_many :words
 end

 sentenza = Sentence.first
 first_word = sentenza.words.first

 first_word.attributes #=> {"label"=>"This", "created_at"=>Wed Apr 07 15:48:30 UTC 2010, "updated_at"=>Wed Apr 07 15:48:30 UTC 2010, "sentence_id"=>"1", "id"=>1, "word_id"=>"1"}

I just want to try to make a deep copy of an ActiveRecord::Base object but I can't use attributes method :

Here is my example :

 words_attributes = sentenza.words.map(&:attributes)

 new_sentenza = Sentenza.create(:label => "2nd Sentence")

 new_sentenza.words.create(words_attributes) but it will fail (since word_id and sentence_id are not present in Word.column_names

Using a safe_attributes method will solve this issue but I am wondering if there is a better way to do it :

def safe_attributes
  attributes.select{|a| self.class.columns_hash[a] }
end

And the call:

 words_attributes = sentenza.words.map(&:safe_attributes)