gistfile1.rb

#!/opt/local/bin/ruby

require 'rubygems'
require 'dm-core'
require 'dm-sweatshop'
require 'dm-aggregates'
require 'pp'

include DataMapper::Sweatshop::Unique

DataMapper.logger = DataMapper::Logger.new($stderr, :debug)
DataMapper.setup(:default, 'mysql://localhost/for_testing')

class Post
  include DataMapper::Resource

  property :id,      Serial
  property :title,    String
  property :user_id, Integer
  
  belongs_to :user
end

class User
  include DataMapper::Resource

  property :id,      Serial
  property :login,    String
  property :role, String
  
  has n, :posts
end

DataMapper.auto_migrate!

User.fixture {{
    :login => unique { /\w+/.gen },
    :role => "dev"
}}

Post.fix {{
  :title => unique { |x| "Cha" * x.to_i },
  :user => User.pick
}}

5.times { User.gen }
10.times { Post.gen }

user_ids = User.all(:role => "admin").map {|u| u.id}
# MysqlError: (mysql_errno=1064, sql_state=42000)
# You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '))' at line 1 Query: SELECT COUNT(*) FROM `posts` WHERE (`user_id` IN ())
Post.count :user_id => user_ids