问如何将现有歌曲添加到音乐库Rails 4

EN

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  belongs_to :meta, polymorphic: true

  before_create :create_empty_profile

  after_create :create_empty_library #may not be the best way to do it ¯\_(ツ)_/¯

  acts_as_messageable

  has_many :playlists

  has_many :user_friendships, dependent: :destroy
  has_many :friends, -> { where(user_friendships: { state: 'accepted'}) }, through: :user_friendships
  has_many :pending_user_friendships, -> { where ({ state: 'pending' }) }, class_name: 'UserFriendship', foreign_key: :user_id
  has_many :pending_friends, through: :pending_user_friendships, source: :friend

  has_many :chat_rooms, dependent: :destroy
  has_many :chat_messages, dependent: :destroy

  has_many :votes, dependent: :destroy

  mount_uploader :profile_pic, ProfilePicUploader

    def mailboxer_name
      self.name
    end

    def mailboxer_email(object)
      self.email
    end

  def admin?
    role == 'admin'
  end

  def moderator?
    role == 'moderator'
  end

  def create_empty_profile
    if is_artist?
      profile = ArtistProfile.new
    else
      profile = UserProfile.new
    end
    profile.save(validate: false)
    self.meta_id = profile.id
    self.meta_type = profile.class.name
  end

  def create_empty_library
    library = Library.new
    library.user_id = self.id
    library.save(validate: false)
  end

end

class Library < ActiveRecord::Base
  belongs_to :user

  has_many :library_songs
  has_many :songs, through: :library_songs

  has_many :library_albums
  has_many :albums, through: :library_albums
end

class LibrarySong < ActiveRecord::Base
  belongs_to :library
  belongs_to :song
end

class LibraryAlbum < ActiveRecord::Base
  belongs_to :library
  belongs_to :album
end

class LibrariesController < ApplicationController
  def index
    @libraries = Library.all
  end

  def show
    @library = Library.find(params[:id])
  end
end

<h1>New Playlist</h1>

<%= form_for(@playlist) do |f| %>
  <%= f.text_field :name %>

  <% Song.all.each do |song| -%>
    <div>
      <%= check_box_tag :song_ids, song.id, false, :name => 'playlist[song_ids][]', id: "song-#{song.id}" %>
      <%= song.name %>
    </div>
  <% end %>

  <%= f.submit %>
<% end %>

class PlaylistsController < ApplicationController
  def index
    @playlists = Playlist.all
  end

  def show
    @playlist = Playlist.find(params[:id])
  end

  def new
    @playlist = Playlist.new
  end

  def create
    @playlist = Playlist.create(playlist_params)
    redirect_to @playlist
  end

  private

  def playlist_params
    params.require(:playlist).permit(:name, song_ids: [])
  end
end