gistfile1.rb

class User < ActiveRecord::Base
  has_many :collection_tracks
  has_many :tracks, through: :collection_tracks
  
  def collection
    tracks
  end
  
  def add_to_collection(track)
    collection_tracks.where(track_id: track.id).first_or_create
  end
  
  def remove_from_collection(track)
    collection_tracks.where(track_id: track.id).try(:destroy)
  end
end

class CollectionTracks < ActiveRecord::Base
  belongs_to :user
  belongs_to :track
end

class Track < ActiveRecord::Base
  has_many :collection_tracks
end