Rails - Feb 2005 - Strange has_and_belongs_to_many behaviour (or is it just me?)

Hi all,

My Rails app is giving an error whenever it tries to add AR objects to  
a collection (has_and_belongs_to_many association between Posts and  
Labels). The application code is something along the lines of  
@post.labels<<collection_of_labels.  The error occurs when Rails tries  
to persist the association by INSERTing a record into the join table.  
What it appears to be trying to do is to insert a value for the primary  
key (`id`) for this table, though this doesn''t make sense as `id` is an
auto_increment column. In all test cases, the value it is trying to add  
to the id column happens to be the same as the foreign key value for  
the record that is being added _to_ the collection, eg:

ActiveRecord::StatementInvalid (Duplicate entry ''5'' for key 1:
INSERT
INTO labels_posts (`post_id`, `id`, `label_id`) VALUES (''8'',
5, ''5'')):

I''ve narrowed down the source of the problem to 2 lines in the  
following file (I''m using the Rails gem). I''ve snipped the
offending
section from the source to give you guys some context. Commenting out  
lines 121-122 resolves the issue.

/usr/local/lib/ruby/gems/1.8/gems/activerecord-1.6.0/lib/active_record/ 
associations/has_and_belongs_to_many_association.rb
118:                when @association_foreign_key
119:                  attributes[column.name] = record.quoted_id
120:                else
121:                  value = record[column.name]
122:                  attributes[column.name] = value unless value.nil?
123:              end
124:              attributes
125:            end

So, is this a bug? What do those two lines do? Could something else be  
causing the error (like my sloppy coding :)? Your thoughts and comments  
are most welcome.

If you need any more information on my setup or code or anything, let  
me know.

Rich Barton

> What it appears to be trying to do is to insert a value for the primary
> key (`id`) for this table, though this doesn''t make sense as `id`
is an
[...]
> ActiveRecord::StatementInvalid (Duplicate entry ''5'' for
key 1: INSERT
> INTO labels_posts (`post_id`, `id`, `label_id`) VALUES
(''8'', 5, ''5'')):
The association table doesn''t need an id field. The post_id and
label_id are quite enough. Make a "compound key" of the two columns
and you should be all set

Jens-Christian

On 9 Feb 2005, at 15:00, Jens-Christian Fischer wrote:
>> What it appears to be trying to do is to insert a value for the 
>> primary
>> key (`id`) for this table, though this doesn''t make sense as
`id` is
>> an
> [...]
>> ActiveRecord::StatementInvalid (Duplicate entry ''5''
for key 1: INSERT
>> INTO labels_posts (`post_id`, `id`, `label_id`) VALUES
(''8'', 5, ''5'')):
>
> The association table doesn''t need an id field. The post_id and 
> label_id are quite enough. Make a "compound key" of the two
columns
> and you should be all set
>
> Jens-Christian
So it was just me then.

Thanks Jens-Christian!

Rich Barton

But what if you want to have a separate primary key, besides the
unique set of the composite foreign keys, in the association table,
because you think you have a good reason or few to want to hide/not
use the composite key for other relationships?


On Wed, 9 Feb 2005 15:08:34 +0100, Rich Barton
<richbarton-ee4meeAH724@public.gmane.org> wrote:
> On 9 Feb 2005, at 15:00, Jens-Christian Fischer wrote:
> 
> >> What it appears to be trying to do is to insert a value for the
> >> primary
> >> key (`id`) for this table, though this doesn''t make sense
as `id` is
> >> an
> > [...]
> >> ActiveRecord::StatementInvalid (Duplicate entry
''5'' for key 1: INSERT
> >> INTO labels_posts (`post_id`, `id`, `label_id`) VALUES
(''8'', 5, ''5'')):
> >
> > The association table doesn''t need an id field. The post_id
and
> > label_id are quite enough. Make a "compound key" of the two
columns
> > and you should be all set
> >
> > Jens-Christian
> 
> So it was just me then.
> 
> Thanks Jens-Christian!
> 
> Rich Barton
> 
> _______________________________________________
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> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
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>

Corey Lawson wrote:
> But what if you want to have a separate primary key, besides the
> unique set of the composite foreign keys, in the association table,
> because you think you have a good reason or few to want to hide/not
> use the composite key for other relationships?
If you''re trying to make the habtm do things it wasn''t
designed to do,
like add lots of attributes or mess around with complex behaviour, then 
90% of the time there''s a good case for creating another model for the 
join table and having two has_many/belongs_to relationships instead of 
one habtm.

Tim.

-- 
Tim Bates
tim-kZbwfhiKUx26c6uEtOJ/EA@public.gmane.org