Rails - May 2006 - destroy failure with has_many associative table without id

Hello,

I''m newbie in RoR development and I''m coping with the
following problem
:

I have two tables "users" and "appmodules", and
I''ve created an
associative table named "permissions" :

  create_table "permissions", :id => false, :force => true do
|t|
    t.column "user_id", :integer, :default => 0, :null => false
    t.column "appmodule_id", :integer, :default => 0, :null =>
false
  end

I''ve explicited disabled the id creation (:id => false), as
"user_id"
and "appmodule_id" are de facto primary keys. the "has_many"
is used for
relationship between user, permission and appmodule.

The problem arises when I want to destroy a user record which has , with 
the following error message :

ActiveRecord::StatementInvalid (Mysql::Error: Unknown column
''id'' in
''where clause'':
DELETE FROM permissions WHERE id = NULL

Any ideas in order to adjust this DELETE statement ? And many thanks in 
advance for the poor newbie like me !

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