将SQL转换为Rails 3 ActiveRecord

Question

I am looking for the most Rails-ish way to retrieve a distinct list of categories for available songs on a given album .

Here it is in SQL for album_id = 1

-- Using subselects
select * from categories where id in (
  select distinct category_id
  from categorizations
  where song_id in (select song_id from album_songs
                    where album_id = 1 and available = 't')
)
order by name asc;

-- Using joins
select distinct c.* from categories c
  inner join categorizations cz on c.id = cz.category_id
  left join album_songs a on cz.song_id = a.song_id
where a.album_id = 1 and a.available = 't'
order by c.name asc;

My working (albeit naive!) attempts to port this to ActiveRecord

## attempting to do it like subselects (although they're not really
## subselects, it executes them individually -- from what i've read
## ActiveRecord won't do subselects?)
Category.where('id IN (?)',
  Categorization.select('DISTINCT category_id').where('song_id IN (?)',
    Album.find(1).songs.available.map(&:song_id)
  ).map(&:category_id)
).order('name ASC')

## joins - although at this point it's pretty much all sql
## as i couldn't find a way to do the left join in pure AR
## i'm also duplicating my AlbumSongs.available scope -- is
## that scope reusable here? (outside the AlbumSongs model?)
Category.select('DISTINCT categories.*')
        .joins(:categorizations,
               'LEFT OUTER JOIN album_songs ON categorizations.song_id = album_songs.song_id')
        .where('album_songs.album_id = ? and available', 1)

I am going with the final one but it seems like I might as well just write it in SQL?

Is there any way to improve this to be more Rails-ish?

Answer 1

Well, it would certainly help if you post your model set up. But assuming that:
* song has_many :categories, :through => categorizations
* an album does not have a huge amount of songs on it

Why not just do:

Album.includes({:songs => {:categorizations => :categories}}).find(1).songs.collect {|s| s.category}.flatten.uniq