category.rb

class Category < ActiveRecord::Base
  has_many :items
  
  validates_presence_of   :name
  validates_uniqueness_of :name
end

gistfile1.txt

Cassady:~/dev/scratch/plugin-testing yossef$ script/console
Loading development environment (Rails 2.0.2)
/>> cat = Category.new
=> #<Category id: nil, name: nil>
>> cat.name = 'testing'
=> "testing"
>> i = Item.new
=> #<Item id: nil, code: nil, category_id: nil>
>> i.code = 'test-1234'
=> "test-1234"
>> i.valid?
=> false
>> i.errors.full_messages
=> ["Category can't be blank"]
>> cat.items << i
=> [#<Item id: nil, code: "test-1234", category_id: nil>]
>> i.valid?
=> false
>> i.errors.full_messages
=> ["Category can't be blank"]
>> cat.save
=> false
>> cat.errors
=> #<ActiveRecord::Errors:0x23d06c4 @base=#<Category id: nil, name: "testing">, @errors={"items"=>["is invalid"]}>
>> exit

item.rb

class Item < ActiveRecord::Base
  belongs_to :category
  
  validates_presence_of :category
  
  validates_presence_of   :code
  validates_uniqueness_of :code
  validates_format_of     :code, :with => /^[a-zA-Z]+-\d+$/
end