has_many :through

01models.rb

class Meeting < ActiveRecord::Base
  has_many :participations, dependent: :destroy
  has_many :players, through: :participations

  belongs_to :place

  attr_accessible :place_id, :name
end

class Participation < ActiveRecord::Base
  belongs_to :meeting
  belongs_to :player

  attr_accessible :meeting_id, :player_id
end
class Place < ActiveRecord::Base
  has_many :meetings

  attr_accessible :address
end

class Player < ActiveRecord::Base
  has_many :participations
  has_many :meetings, through: :participations
  

  attr_accessible :name
end

02has_many_through_tt.log.rb

1.9.3-p125 :005 > john=Player.create(name:'John')
   (0.1ms)  begin transaction
  SQL (112.6ms)  INSERT INTO "players" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", Thu, 27 Sep 2012 20:48:12 UTC +00:00], ["name", "John"], ["updated_at", Thu, 27 Sep 2012 20:48:12 UTC +00:00]]
   (154.4ms)  commit transaction
 => #<Player id: 1, name: "John", created_at: "2012-09-27 20:48:12", updated_at: "2012-09-27 20:48:12"> 
1.9.3-p125 :006 > Place.create(address: 'central park')
   (0.1ms)  begin transaction
  SQL (0.6ms)  INSERT INTO "places" ("address", "created_at", "updated_at") VALUES (?, ?, ?)  [["address", "central park"], ["created_at", Thu, 27 Sep 2012 20:48:33 UTC +00:00], ["updated_at", Thu, 27 Sep 2012 20:48:33 UTC +00:00]]
   (128.2ms)  commit transaction
 => #<Place id: 1, address: "central park", created_at: "2012-09-27 20:48:33", updated_at: "2012-09-27 20:48:33"> 
1.9.3-p125 :007 > john.meetings.create(name:'PvP,5x2,3/4', place_id: Place.first.id).participations.create
  Place Load (0.2ms)  SELECT "places".* FROM "places" LIMIT 1
   (0.1ms)  begin transaction
  SQL (0.5ms)  INSERT INTO "meetings" ("created_at", "name", "place_id", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00], ["name", "PvP,5x2,3/4"], ["place_id", 1], ["updated_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00]]
  SQL (0.5ms)  INSERT INTO "participations" ("created_at", "meeting_id", "player_id", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00], ["meeting_id", 1], ["player_id", 1], ["updated_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00]]
   (112.8ms)  commit transaction
   (0.1ms)  begin transaction
  SQL (0.5ms)  INSERT INTO "participations" ("created_at", "meeting_id", "player_id", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00], ["meeting_id", 1], ["player_id", nil], ["updated_at", Thu, 27 Sep 2012 20:48:42 UTC +00:00]]
   (108.5ms)  commit transaction
 => #<Participation id: 2, meeting_id: 1, player_id: nil, created_at: "2012-09-27 20:48:42", updated_at: "2012-09-27 20:48:42"> 
1.9.3-p125 :008 > Participation.all
  Participation Load (0.3ms)  SELECT "participations".* FROM "participations" 
 => [#<Participation id: 1, meeting_id: 1, player_id: 1, created_at: "2012-09-27 20:48:42", updated_at: "2012-09-27 20:48:42">, #<Participation id: 2, meeting_id: 1, player_id: nil, created_at: "2012-09-27 20:48:42", updated_at: "2012-09-27 20:48:42">]