Question

  render :json => {
    "playlist" => playlist_description,
    "songs" => @playlist.songs.as_json(:include => {:playlist_songs => {:only => [:id, :position]}})
  }

The above code results in 1+N queries to the database, one to load playlist_songs for each song.
The playlist is preloaded in @playlist.

This is so slow, how can I optimize?

Answer 1

My guess: You’re not eager-loading the playlist_songs at the moment. You’re currently waiting until the as_json call – after which all the songs have been loaded – and then the code has to iterate over every song and fetch the playlist_songs then.

My guess (this is totally untested and may include bugs)

@playlist.songs.all(:include => :playlist_songs).as_json(:include => {:playlist_songs => {:only => [:id, :position]}})

AFAICT, this should first eager load all the songs and the playlist_songs… and then render as json.