Snippet from lesson, where we came up with the migrations and models.

migrations_and_models.rb

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

  create_table :playlists do |t|    
    t.string  :name
    t.integer :currently_playing_id
  end
  
  create_table :playlist_song do |t|
    t.integer :playlist_id
    t.integer :song_id
  end
  
  create_table :songs do |t|
    t.string :title
    t.string :artist
    t.string :soundcloud_url
  end
end

class PlaylistSong < ActiveRecord::Base
  self.table_name = :playlist_song
  
  belongs_to :song
  belongs_to :playlist
end

class Playlist < ActiveRecord::Base
  has_many :playlist_songs
  has_many :songs, through: :playlist_songs
  belongs_to :currently_playing, class_name: 'Song'
end

class Song < ActiveRecord::Base
  has_many :playlist_songs
  has_many :playlists, through: :playlist_songs
end



our_love = autumn_leaves = nil

playlists = [
  Playlist.create! do |pl|
    pl.name = 'Jazz'
    our_love = pl.songs.build title:          "Our Love Is Here To Stay",
                              artist:         "Ella Fitzgerald",
                              soundcloud_url: "FILL ME IN WHEN WE GET HERE!"

    autumn_leaves = pl.songs.build title:          "Autumn Leaves",
                                   artist:         "Chet Baker",
                                   soundcloud_url: "FILL ME IN WHEN WE GET HERE!"

    pl.currently_playing = our_love
  end,
  Playlist.create!(name: 'Rock'),
  Playlist.create!(name: 'Nu Metal'),
]


playlists
# => [#<Playlist:0x007ffaf1a1cd88
#      id: 1,
#      name: "Jazz",
#      currently_playing_id: 1>,
#     #<Playlist:0x007ffaf3218900
#      id: 2,
#      name: "Rock",
#      currently_playing_id: nil>,
#     #<Playlist:0x007ffaf3222310
#      id: 3,
#      name: "Nu Metal",
#      currently_playing_id: nil>]

Song.all
# => [#<Song:0x007ffaf1d3b1e0
#      id: 1,
#      title: "Our Love Is Here To Stay",
#      artist: "Ella Fitzgerald",
#      soundcloud_url: "FILL ME IN WHEN WE GET HERE!">,
#     #<Song:0x007ffaf1d3b078
#      id: 2,
#      title: "Autumn Leaves",
#      artist: "Chet Baker",
#      soundcloud_url: "FILL ME IN WHEN WE GET HERE!">]

our_love.playlists
# => [#<Playlist:0x007ffaf1d6b5c0
#      id: 1,
#      name: "Jazz",
#      currently_playing_id: 1>]

autumn_leaves.playlists
# => [#<Playlist:0x007ffaf1d72758
#      id: 1,
#      name: "Jazz",
#      currently_playing_id: 1>]

playlists.map(&:songs)
# => [[#<Song:0x007ffaf1d03ee8
#       id: 1,
#       title: "Our Love Is Here To Stay",
#       artist: "Ella Fitzgerald",
#       soundcloud_url: "FILL ME IN WHEN WE GET HERE!">,
#      #<Song:0x007ffaf3198110
#       id: 2,
#       title: "Autumn Leaves",
#       artist: "Chet Baker",
#       soundcloud_url: "FILL ME IN WHEN WE GET HERE!">],
#     [],
#     []]

# >> D, [2015-06-11T14:38:08.576653 #73764] DEBUG -- :    (0.3ms)  CREATE TABLE "playlists" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar, "currently_playing_id" integer) 
# >> D, [2015-06-11T14:38:08.577017 #73764] DEBUG -- :    (0.1ms)  CREATE TABLE "playlist_song" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "playlist_id" integer, "song_id" integer) 
# >> D, [2015-06-11T14:38:08.577303 #73764] DEBUG -- :    (0.1ms)  CREATE TABLE "songs" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "title" varchar, "artist" varchar, "soundcloud_url" varchar) 
# >> D, [2015-06-11T14:38:08.646873 #73764] DEBUG -- :    (0.1ms)  begin transaction
# >> D, [2015-06-11T14:38:08.653816 #73764] DEBUG -- :   SQL (0.2ms)  INSERT INTO "songs" ("title", "artist", "soundcloud_url") VALUES (?, ?, ?)  [["title", "Our Love Is Here To Stay"], ["artist", "Ella Fitzgerald"], ["soundcloud_url", "FILL ME IN WHEN WE GET HERE!"]]
# >> D, [2015-06-11T14:38:08.654926 #73764] DEBUG -- :   SQL (0.1ms)  INSERT INTO "playlists" ("name", "currently_playing_id") VALUES (?, ?)  [["name", "Jazz"], ["currently_playing_id", 1]]
# >> D, [2015-06-11T14:38:08.659181 #73764] DEBUG -- :   SQL (0.1ms)  INSERT INTO "playlist_song" ("playlist_id", "song_id") VALUES (?, ?)  [["playlist_id", 1], ["song_id", 1]]
# >> D, [2015-06-11T14:38:08.659886 #73764] DEBUG -- :   SQL (0.0ms)  INSERT INTO "songs" ("title", "artist", "soundcloud_url") VALUES (?, ?, ?)  [["title", "Autumn Leaves"], ["artist", "Chet Baker"], ["soundcloud_url", "FILL ME IN WHEN WE GET HERE!"]]
# >> D, [2015-06-11T14:38:08.661082 #73764] DEBUG -- :   SQL (0.0ms)  INSERT INTO "playlist_song" ("playlist_id", "song_id") VALUES (?, ?)  [["playlist_id", 1], ["song_id", 2]]
# >> D, [2015-06-11T14:38:08.661449 #73764] DEBUG -- :    (0.1ms)  commit transaction
# >> D, [2015-06-11T14:38:08.661836 #73764] DEBUG -- :    (0.1ms)  begin transaction
# >> D, [2015-06-11T14:38:08.662303 #73764] DEBUG -- :   SQL (0.1ms)  INSERT INTO "playlists" ("name") VALUES (?)  [["name", "Rock"]]
# >> D, [2015-06-11T14:38:08.662564 #73764] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2015-06-11T14:38:08.662768 #73764] DEBUG -- :    (0.0ms)  begin transaction
# >> D, [2015-06-11T14:38:08.663268 #73764] DEBUG -- :   SQL (0.1ms)  INSERT INTO "playlists" ("name") VALUES (?)  [["name", "Nu Metal"]]
# >> D, [2015-06-11T14:38:08.663537 #73764] DEBUG -- :    (0.0ms)  commit transaction
# >> D, [2015-06-11T14:38:08.664806 #73764] DEBUG -- :   Song Load (0.1ms)  SELECT "songs".* FROM "songs"
# >> D, [2015-06-11T14:38:08.674902 #73764] DEBUG -- :   Playlist Load (0.2ms)  SELECT "playlists".* FROM "playlists" INNER JOIN "playlist_song" ON "playlists"."id" = "playlist_song"."playlist_id" WHERE "playlist_song"."song_id" = ?  [["song_id", 1]]
# >> D, [2015-06-11T14:38:08.676128 #73764] DEBUG -- :   Playlist Load (0.0ms)  SELECT "playlists".* FROM "playlists" INNER JOIN "playlist_song" ON "playlists"."id" = "playlist_song"."playlist_id" WHERE "playlist_song"."song_id" = ?  [["song_id", 2]]
# >> D, [2015-06-11T14:38:08.677823 #73764] DEBUG -- :   Song Load (0.1ms)  SELECT "songs".* FROM "songs" INNER JOIN "playlist_song" ON "songs"."id" = "playlist_song"."song_id" WHERE "playlist_song"."playlist_id" = ?  [["playlist_id", 1]]
# >> D, [2015-06-11T14:38:08.679345 #73764] DEBUG -- :   Song Load (0.0ms)  SELECT "songs".* FROM "songs" INNER JOIN "playlist_song" ON "songs"."id" = "playlist_song"."song_id" WHERE "playlist_song"."playlist_id" = ?  [["playlist_id", 2]]
# >> D, [2015-06-11T14:38:08.679979 #73764] DEBUG -- :   Song Load (0.1ms)  SELECT "songs".* FROM "songs" INNER JOIN "playlist_song" ON "songs"."id" = "playlist_song"."song_id" WHERE "playlist_song"."playlist_id" = ?  [["playlist_id", 3]]