gistfile1.rbx

#!/usr/bin/env ruby
require 'rubygems'
require 'dm-core'
require 'dm-is-list'
require 'pp'

class Participation
  include DataMapper::Resource

  property :id, Serial

  belongs_to :event
  belongs_to :client
  
  is :list, :scope => [:event_id]
end

class Event
  include DataMapper::Resource
  
  property :id, Serial
  property :name, String

  has n, :participations, :order => [:position.asc]
  has n, :clients, :through => :participations
end

class Client
  include DataMapper::Resource

  property :id, Serial
  property :name, String
  
  has n, :participations
  has n, :events, :through => :participations
end

#DataMapper.logger.set_log($stderr, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')
DataMapper.auto_migrate!()

e1 = Event.create(:name => 'Event1')
e2 = Event.create(:name => 'Event2')

c1 = Client.create(:name => 'Client1')
c2 = Client.create(:name => 'Client2')

p1 = Participation.create(:client => c1, :event => e1)
p2 = Participation.create(:client => c2, :event => e1)
p3 = Participation.create(:client => c1, :event => e2)

pp e1.clients
#pp e2.clients

#puts p1.position ,p2.position, p3.position
p2.move :highest

e1.reload
e1.clients.reload
pp e1.clients
pp e1.participations.map { |p| p.client }
#p1.reload
#p2.reload
#p3.reload

#puts p1.position, p2.position, p3.position