Has and belongs to many, join table id included in query

I'm using a habtm association for the first time and I'm having some
trouble when inserting several rows. The id of the join table seems to
be set to an already existing id causing the insert to fail.

class ClassA < ActiveRecord::Base
  has_and_belongs_to_many :bs,
                          :class_name => "ClassB",
                          :join_table => :abs

end

class ClassB < ActiveRecord::Base
  belongs_to :class_a
end

My join table migration:

  def self.up
    create_table :abs do |t|
      t.integer :class_a_id, :null => false
      t.integer :class_b_id, :null => false
    end

    add_index :abs, [:class_a_id, :class_b_id], :unique => true
  end

Then finally in the controller:

@a.bs = ClassB.find params[:bs]
@a.save

Now, this works the first time, but the second time I get this back:

Mysql::Error: #23000Duplicate entry '1' for key 1: INSERT INTO `abs`
(`class_a_id`, `id`, `class_b_id`) VALUES (7, 1, 1)

Now, I wonder - why is the id included in the query with a value that is
already used? I'm sure it's just something I'm doing wrong, but I'm
having a hard time figuring out what. I looked at the :insert_sql option
to habtm too, but don't know how to include the ids dynamically?

I'm using a habtm association for the first time and I'm having some
trouble when inserting several rows. The id of the join table seems to
be set to an already existing id causing the insert to fail.

habtm join tables should not have an id column (use :id => false as an
option to create_table)

Fred

Not to pimp my blog or anything, but I wrote about this exact issue
here: http://bkocik.net/2007/07/26/habtm-and-the-id-field/

That should help you understand what's going on (in a very long-winded
manner). :slight_smile:

Bill Kocik wrote:

Not to pimp my blog or anything, but I wrote about this exact issue
here: http://bkocik.net/2007/07/26/habtm-and-the-id-field/

That should help you understand what's going on (in a very long-winded
manner). :slight_smile:

On Oct 14, 6:26�pm, Christian Johansen <rails-mailing-l...@andreas-

Ah, great stuff, thanks!