[ruby-it] Active record

[joxer]

Sto giochicchiando con active record e provavo le associazioni.
Dovrei provare a trovare dentro la table books la colonna "username" 
dalla tabella   Item da cui dipende.

L'errore è questo:

joxer a polly:~/ruby/activerecord$ ruby mysql_association.rb
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/abstract_adapter.rb:147:in 
`log': Mysql::Error: Unknown column 'books.item_id' in 'where clause': 
SELECT * FROM `books`     WHERE (`books`.item_id = 1)  
(ActiveRecord::StatementInvalid)
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/mysql_adapter.rb:299:in 
`execute'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/mysql_adapter.rb:515:in 
`select'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:7:in 
`select_all_without_query_cache'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/abstract/query_cache.rb:61:in 
`select_all'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:582:in 
`find_by_sql'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:1341:in 
`find_every'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:536:in 
`find'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:47:in 
`find'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:305:in 
`find_target'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:259:in 
`load_target'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_proxy.rb:173:in 
`method_missing'
    from 
/var/lib/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:274:in 
`method_missing'
    from mysql_association.rb:21

il codice riguardante :

class Item < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :item
end

a = Item.find( :first )
a.books.each do |s|
        s.username
end

Scusate la niubbagine ma non ho neanche trovato buoni tutorial in rete e 
ho visto perlopiù dalle doc.
Grazie,  joxer