Rails Order by Contained Objects 2 Levels Deep?

Question

How can I include ordering in an 'order' ActiveRelation call that's more than one level deep?

That is, I understand the answer when it's only one level deep (asked and answered at Rails order by associated data ). However, I have a case where the data on which I want to sort is two levels deep.

Specifically, in my schema a SongbookEntry contains a Recording, which contains an Artist and a Song. I want to be able to sort SongbookEntry lists by song title.

I can go one level deep and sort Recordings by song title:

@recordings = Recording.includes(:song).order('songs.title')

...but don't know how to go two levels deep. In addition, it would be great if I could sort on the recording (that is, the song title and the artist name) -- is this possible without descending into SQL?

Thanks for any help,

Keith

Answer 1

我建议将歌手姓名（可能还有歌曲名称）存储在录音本身上，因此您不必“降级到SQL”。

Answer 2

Try this
SongbookEntry.includes(:recording=>[:artist,:song]).order('songs.title, artists.name')

You can use joins in place of includes if you don't want to use associated tables fields in views

Answer 3

If you model the association between SongbookEntry and Song as such:

class SongbookEntry < ActiveRecord::Base
  # ...
  has_one :song, through: :recording
end

you will be able to access @songbookentry.song and SongbookEntry.joins(:song) using your existing schema.

Edit:

Applying the same idea for Artist , a possible query would be:

SongbookEntry.joins(:song,:artist).order('songs.title','artists.name')

Note that this may not be the most efficient operation (multiple joins involved) even though it looks Rails-ish, so later on you may want to denormalize the tables as Ryan suggested, or find another way to model the data.