0
我正在寻找最大的Rails-ish方式来检索给定专辑上的可用歌曲的不同类别列表。将SQL转换为Rails 3 ActiveRecord
这是SQL的album_id = 1个
-- 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;
我的工作（尽管天真！）尝试端口这ActiveRecord的
## 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)
我与最后一个去，但它似乎像我可能只是写在SQL中？
有没有什么办法可以改善这一点？Rails-ish？