Rails ActiveRecord: how to get non-duplicate child records by super-parent in view?

问题

I have four tables called Albums, Songs, Singers and SongSingers.

Albums { Id, Name } 
Songs { Id, AlbumId, Name }
Singers { Id, Name }
SongSingers { Id, SongId, SingerId }

Album has many songs.
Song belongs to an Album.
SongSinger belongs to Song and Singer.
Song and Singer have many SongSingers.

Each song may have same or different singers.

In View, how can I get all non-duplicate Singers by AlbumId.

Thanks.

回答1:

Absolutely no problem.

You are querying for singers. That's what the query will start with.
You have a condition on songs stored in association. We'll need it soon.
```
@album.songs
```
Join songs and singers so you can apply the above condition. Note: you're still selecting singers! It's just that now you can place conditions on their songs.
```
Singer.joins(:songs)
```

Now apply it:

Singer.joins(:songs).merge(@album.songs)
# It's pretty much the same as this, but more concise:
Singer.joins(:songs).merge(songs: {album_id: @album.id})

Now filter down duplicates:

Singer.joins(:songs).merge(@album.songs).uniq

Done.

回答2:

You can do this.

class Album < ActiveRecord::Base

  has_many :songs
  has_many :song_singers, :through => :songs
  has_many :singers,      :through => :song_singers

  ...

end

In the view, you can now simply call this

@album.singers.uniq

来源：https://stackoverflow.com/questions/30544385/rails-activerecord-how-to-get-non-duplicate-child-records-by-super-parent-in-vi

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ruby-on-rails

ruby

activerecord

rails-activerecord