2007-08-08

(HABTM) has_and_belongs_to_many Bug in Ruby On Rails

Once the HABTM relation was made, for example:

class Song &lt; ActiveRecord::Base
  has_and_belongs_to_many :artists
end
`</pre>
The operation shown below is valid:
<pre>`song.artists &lt;&lt; Artist.new(:name =&gt; 'Barbra Streisand')
`</pre>
But this may make a BIG problem:
<pre>`song.artists &lt;&lt; Artist.find(:name =&gt; 'Barbra Streisand')

Mysql::Error: Duplicate entry '1' for key 1: INSERT INTO artists_songs (`artist_id`, `id`, `song_id`) VALUES (1, 1, 3)
`</pre>
The `id` field should be "autoincrement". Why pass a Fixnum to it? That is the question. I find out that when we push(&lt;&lt;) an artist to a song, whole the attributes on this artist will  be expanded as arguments of the SQL query. And then duplicate :id to :artist_id.

Hum, it sounds great. There is no problem. _But_ the :id key-value pair is still. That means **we will establish a HABTM relation with a given key**. So we sometimes get error from here. Try the tricky way:
<pre>`class Song &lt; ActiveRecord::Base
  has_and_belongs_to_many :artists,
  :insert_sql =&gt; 'INSERT INTO `artists_songs` ( `song_id`, `artist_id` )
                  VALUES (#{id}, #{record.id})'
end

How awful the code!