Question

I have the following models:

• User
• Playlist
• SongLike
• Song
• …

When I query a playlist for all of its songs, I get an array of song objects returned. What’s the most efficient way to find out which of these songs a user has “liked”? The likes are stored in the SongLike model:

class SongLike < ActiveRecord::Base
  belongs_to :user
  belongs_to :song, :counter_cache => "likes_count"
end

…this is the song model:

class Song < ActiveRecord::Base
  has_and_belongs_to_many :playlists
  has_many :featured_songs
  has_many :song_likes
  has_many :users, :through => :song_likes
  ...
end

Answer 1

Utilize your song_likes association along with the songs in memory rather than querying the db multiple times:

# user.rb
def songs_liked_from(subset)
  like_ids = self.song_likes.where(:song_id => subset.map(&:id)).select(:song_id).map(&:song_id)
  subset.select{|s| liked_ids.include?(s.id) }
end