category.rb

class Category
  include DataMapper::Resource
  
  property :id,           Serial
  property :name,         String
  property :description,  Text
  property :url,          String
  property :created_at,   DateTime
  property :updated_at,   DateTime
  
  has n, :images
  has n, :users,  :through => Resource

end

console.txt

# using merb console

>> user = User.get(1)
 ~ SELECT `id`, `login`, `email`, `image`, `created_at`, `updated_at`, `crypted_password`, `salt` FROM `users` WHERE (`id` = 1) ORDER BY `id` LIMIT 1
=> #<User id=1 login="mitkok" email=nil description=<not loaded> image=nil created_at=#<DateTime: 13255947691/5400,1/12,2299161> updated_at=#<DateTime: 13255947691/5400,1/12,2299161> crypted_password="132dd5baa9b6f8a6b2e85b3fd439aac99263f85d" salt="11a0b1b5cd533613d8b961d18ad5276ca865d8d9">

>> user.categories
 ~ SELECT `categories`.`id`, `categories`.`name`, `categories`.`url`, `categories`.`created_at`, `categories`.`updated_at` FROM `categories` INNER JOIN `categories_users` ON (`categories`.`id` = `categories_users`.`category_id`) WHERE (`categories_users`.`user_id` = 1) GROUP BY `categories`.`id`, `categories`.`name`, `categories`.`url`, `categories`.`created_at`, `categories`.`updated_at` ORDER BY `categories`.`id`
=> []

>> cat = Category.new
=> #<Category id=nil name=nil description=nil url=nil created_at=nil updated_at=nil>

>> cat.name = 'Homeless dogs'
=> "Homeless dogs" 

>> cat.description = 'Really dirty and dangerous!'
=> "Really dirty and dangerous!"

>> user.categories << cat
 ~ INSERT INTO `categories` (`name`, `updated_at`, `description`, `created_at`) VALUES ('Homeless dogs', '2008-12-05 15:26:08', 'Really dirty and dangerous!', '2008-12-05 15:26:08')
 ~ SELECT `category_id`, `user_id` FROM `categories_users` WHERE (`user_id` IN (1)) ORDER BY `category_id`, `user_id`
=> [#<Category id=3 name="Homeless dogs" description="Really dirty and dangerous!" url=nil created_at=#<DateTime: 13255952723/5400,1/12,2299161> updated_at=#<DateTime: 13255952723/5400,1/12,2299161>>]

# Here category is created, but the association in table categories_users not.

user.rb

class User
  include DataMapper::Resource
  
  # Attributes
        property :id,               Serial
        property :login,            String
        property :email,            String
        property :description,      Text
        property :image,            String
        property :created_at,       DateTime
        property :updated_at,       DateTime

        # Associations
        has n, :categories, :through => Resource

        # Validations
        validates_present   :login, :password, :password_confirmation
        validates_is_unique :login
  
end