Rails - Jun 2005 - saving a habtm, why "update"?

Can someone help me understand this?  I have a habtm between Books and
Authors.  In my books form, I added a form field that is a bunch of
check boxes with a list of authors, where they are named like:

name=''authors_books["author_id"][id of this author]''

I added the following function to my Books model:

 def authors=( authors_form_hsh )
        self.authors.each{ |aa| self.authors.delete( aa ) }
        authors_form_hsh.each_key{ |key| self.authors << Author.find( key
) }
    end

And in my books controller:

 def create
        @book = Book.new(@params[:book])
        @book.authors =
@params["authors_books"]["author_id"]   # <--
I added this to the scaffold
        if @book.save
          flash[''notice''] = ''Book was successfully
created.''
          redirect_to :action => ''list''
        else
          render_action ''new''
        end
    end

So, I am passing the hash to authors=(), and for every key in that
hash(the author id''s), I add a relationship.

It seems to be working, *but*, my log shows me something that has me
concerned: (i''ve snipped some of the content to make the log shorter)

Processing BooksController#create (for 128.249.153.72 at Tue Jun 14
15:07:41 MST 2005)
  Parameters:
{"authors_books"=>{"author_id"=>{"1"=>"",
"2"=>""}},
"submit"=>"Create",
"action"=>"create",
"controller"=>"books",
"book"=>{"date(1i)"=>"2005",
"date(2i)"=>"6", "title"=>"Test
Book6",
"date(3i)"=>"14",
"series_index"=>"37", "cover"=>"",
"about"=>"testing, 123",
"publisher"=>"blah",
"series_id"=>"2"}}
  Book Columns (0.001261)   SHOW FIELDS FROM books
  Book Columns (0.001186)   SHOW FIELDS FROM books
{"1"=>"", "2"=>""}
  Author Load (0.001013)   SELECT * FROM authors WHERE authors.id =
''1'' LIMIT 1
  SQL (0.000245)   BEGIN
  Author Columns (0.001011)   SHOW FIELDS FROM authors
  SQL (0.000237)   COMMIT
  Author Load (0.000892)   SELECT * FROM authors WHERE authors.id =
''2'' LIMIT 1
  SQL (0.000240)   BEGIN
  SQL (0.000171)   COMMIT
  SQL (0.000183)   BEGIN
  SQL (0.000596)   INSERT INTO books (`title`, `date`, `series_index`,
`cover`, `about`, `publisher`, `series_id`) VALUES(''Test
Book6'',
''2005-06-14'', 37, '''', ''testing,
123'', ''blah'', 2)
  Author Update (0.000736)   UPDATE authors SET (*snip* every field is
updated, but with apparently the same content) WHERE id = 1
  authors_books Columns (0.000957)   SHOW FIELDS FROM authors_books
  SQL (0.000409)   INSERT INTO authors_books (`author_id`, `book_id`)
VALUES (''1'', ''171'')
  Author Update (0.000583)   UPDATE authors SET (*snip* every field is
updated, but with apparently the same content) WHERE id = 2
  authors_books Columns (0.000939)   SHOW FIELDS FROM authors_books
  SQL (0.000403)   INSERT INTO authors_books (`author_id`, `book_id`)
VALUES (''2'', ''171'')
  SQL (0.000255)   COMMIT


The statements that have me concerned are the "UPDATE authors SET"
statements.  All I have done (or so I thought), was create a new book
and added an entry in the authors_books table.  Why on earth are both
of the authors being "updated" (these are the 2 authors of this book)?
 Their content should not be changing.  Did I do something wrong?

Matt

Belorion <belorion-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> I added the following function to my Books model:
>
>  def authors=( authors_form_hsh )
>         self.authors.each{ |aa| self.authors.delete( aa ) }
>         authors_form_hsh.each_key{ |key| self.authors << Author.find(
key ) }
>     end
I''d do self.authors.clear instead of the each...delete iterator.  Note
though that there are more efficient implementations of this scheme
than just "clear and add all".  This really needs a well defined home
on the wiki for easy searching.
> The statements that have me concerned are the "UPDATE authors
SET"
> statements.  All I have done (or so I thought), was create a new book
> and added an entry in the authors_books table.  Why on earth are both
> of the authors being "updated" (these are the 2 authors of this
book)?
>  Their content should not be changing.  Did I do something wrong?
Look at the bullet list titled "Collections"  on at the top of the
docs for
ActiveRecord::Associations::ClassMethods.  Each of the objects pushed
onto the collection are going to get saved when the containing object
are saved.  I think the reasoning is that if you have an updated_at or
updated_on column it needs to reflect that the object was added to a
container.  

If you use the self.authors.build method I don''t think the individual
authors would be updated.  However, looking at the docs for
has_and_belongs_to_many, it doesn''t look like the collection has a
build method; only has_many.  So you may be stuck always getting an
update on your individual author fields.
-- 
doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org