Many to Many

example_of_many_to_many_in_active_record.rb

# https://github.com/BrantDFaulkner/database_schema
require 'active_record'
require 'logger'

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Base.logger = Logger.new $stdout
ActiveSupport::LogSubscriber.colorize_logging = false

ActiveRecord::Schema.define do
  self.verbose = false
  
  # MIGRATION
  create_table :teams do |t|
    t.string :name
  end
  create_table :fans do |t|
    t.string :name
  end
  create_table :fans_teams do |t|
    t.integer :team_id
    t.integer :fan_id
  end
end


# MODELS
class Team < ActiveRecord::Base
  has_many :fans_team
  has_many :fans, through: :fans_team
end
class Fan < ActiveRecord::Base
  has_many :fans_team
  has_many :teams, through: :fans_team
end
class FansTeam < ActiveRecord::Base
  belongs_to :team
  belongs_to :fan
end

# SEEDS (or tests or controller or console or w/e)
white_socks = Team.create! name: 'White Socks'
cubs        = Team.create! name: 'Cubs'

Fan.create! name: 'Jim',     teams: [white_socks, cubs]
Fan.create! name: 'Sally',   teams: [white_socks]
Fan.create! name: 'Charlie', teams: [cubs]
Team.first # => #<Team id: 1, name: "White Socks">
    .fans  # => #<ActiveRecord::Associations::CollectionProxy [#<Fan id: 1, name: "Jim">, #<Fan id: 2, name: "Sally">]>
    .last  # => #<Fan id: 2, name: "Sally">
    .teams # => #<ActiveRecord::Associations::CollectionProxy [#<Team id: 1, name: "White Socks">]>
    .first # => #<Team id: 1, name: "White Socks">
    .fans  # => #<ActiveRecord::Associations::CollectionProxy [#<Fan id: 1, name: "Jim">, #<Fan id: 2, name: "Sally">]>
    .last  # => #<Fan id: 2, name: "Sally">
    .name  # => "Sally"

# >> D, [2016-04-29T11:10:35.881889 #94256] DEBUG -- :    (0.3ms)  CREATE TABLE "teams" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar) 
# >> D, [2016-04-29T11:10:35.882344 #94256] DEBUG -- :    (0.1ms)  CREATE TABLE "fans" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar) 
# >> D, [2016-04-29T11:10:35.882613 #94256] DEBUG -- :    (0.1ms)  CREATE TABLE "fans_teams" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "team_id" integer, "fan_id" integer) 
# >> D, [2016-04-29T11:10:35.903784 #94256] DEBUG -- :    (0.0ms)  begin transaction
# >> D, [2016-04-29T11:10:35.906699 #94256] DEBUG -- :   SQL (0.1ms)  INSERT INTO "teams" ("name") VALUES (?)  [["name", "White Socks"]]
# >> D, [2016-04-29T11:10:35.906995 #94256] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2016-04-29T11:10:35.907203 #94256] DEBUG -- :    (0.0ms)  begin transaction
# >> D, [2016-04-29T11:10:35.907545 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "teams" ("name") VALUES (?)  [["name", "Cubs"]]
# >> D, [2016-04-29T11:10:35.907758 #94256] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2016-04-29T11:10:35.936549 #94256] DEBUG -- :    (0.1ms)  begin transaction
# >> D, [2016-04-29T11:10:35.937435 #94256] DEBUG -- :   SQL (0.1ms)  INSERT INTO "fans" ("name") VALUES (?)  [["name", "Jim"]]
# >> D, [2016-04-29T11:10:35.938180 #94256] DEBUG -- :   SQL (0.1ms)  INSERT INTO "fans_teams" ("team_id", "fan_id") VALUES (?, ?)  [["team_id", 1], ["fan_id", 1]]
# >> D, [2016-04-29T11:10:35.938819 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "fans_teams" ("team_id", "fan_id") VALUES (?, ?)  [["team_id", 2], ["fan_id", 1]]
# >> D, [2016-04-29T11:10:35.939675 #94256] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2016-04-29T11:10:35.940784 #94256] DEBUG -- :    (0.0ms)  begin transaction
# >> D, [2016-04-29T11:10:35.941198 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "fans" ("name") VALUES (?)  [["name", "Sally"]]
# >> D, [2016-04-29T11:10:35.941676 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "fans_teams" ("team_id", "fan_id") VALUES (?, ?)  [["team_id", 1], ["fan_id", 2]]
# >> D, [2016-04-29T11:10:35.942121 #94256] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2016-04-29T11:10:35.942967 #94256] DEBUG -- :    (0.0ms)  begin transaction
# >> D, [2016-04-29T11:10:35.943359 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "fans" ("name") VALUES (?)  [["name", "Charlie"]]
# >> D, [2016-04-29T11:10:35.943835 #94256] DEBUG -- :   SQL (0.0ms)  INSERT INTO "fans_teams" ("team_id", "fan_id") VALUES (?, ?)  [["team_id", 2], ["fan_id", 3]]
# >> D, [2016-04-29T11:10:35.944287 #94256] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2016-04-29T11:10:35.945192 #94256] DEBUG -- :   Team Load (0.1ms)  SELECT  "teams".* FROM "teams"  ORDER BY "teams"."id" ASC LIMIT 1
# >> D, [2016-04-29T11:10:35.956820 #94256] DEBUG -- :   Fan Load (0.1ms)  SELECT "fans".* FROM "fans" INNER JOIN "fans_teams" ON "fans"."id" = "fans_teams"."fan_id" WHERE "fans_teams"."team_id" = ?  [["team_id", 1]]
# >> D, [2016-04-29T11:10:35.958233 #94256] DEBUG -- :   Team Load (0.1ms)  SELECT "teams".* FROM "teams" INNER JOIN "fans_teams" ON "teams"."id" = "fans_teams"."team_id" WHERE "fans_teams"."fan_id" = ?  [["fan_id", 2]]
# >> D, [2016-04-29T11:10:35.958912 #94256] DEBUG -- :   Fan Load (0.1ms)  SELECT "fans".* FROM "fans" INNER JOIN "fans_teams" ON "fans"."id" = "fans_teams"."fan_id" WHERE "fans_teams"."team_id" = ?  [["team_id", 1]]