Rails：急切地加载as_json包括 [英] Rails: Eager loading as_json includes

问题描述

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

上面的代码对数据库产生1 + N个查询，每个查询都会加载一首歌的playlist_songs。
播放列表已预加载到@playlist中。

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.

这太慢了，如何优化？

推荐答案

我的猜测：您现在不急于加载playlist_songs。您当前正在等待as_json调用（此后所有歌曲均已加载），然后代码必须遍历每首歌曲，然后获取playlist_song。

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.

我的猜测（完全未经测试，并且可能包含错误）

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，这应该首先渴望加载所有歌曲和 playlist_songs ...然后呈现为json。

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

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