Losing an Attribute When Saving Through an Association w/ Scope (Rails 4.0.0)

问题

The Code (Rails 4.0.0)

class Track < ActiveRecord::Base
  has_many :artist_tracks
  has_many :owning_artists,
              -> { where(:artist_tracks => { :artistic_role_id => 1 }) },
              :through => :artist_tracks,
              :source => :artist
end

class ArtistTrack < ActiveRecord::Base
  belongs_to :artist
  belongs_to :track
  belongs_to :artistic_role
end

class Artist < ActiveRecord::Base
  has_many :artist_tracks
  has_many :tracks, :through => :artist_tracks
end

Finding Works

# artist_tracks.artistic_role_id is properly set to "1"
2.0.0p195 :003 > Track.last.owning_artists

  Track Load (1.1ms)  SELECT "tracks".* FROM "tracks" ORDER BY "tracks"."id" DESC LIMIT 1
  Artist Load (0.8ms)  SELECT "artists".* FROM "artists" INNER JOIN "artist_tracks" ON "artists"."id" = "artist_tracks"."artist_id" WHERE "artist_tracks"."artistic_role_id" = 1 AND "artist_tracks"."track_id" = $1  [["track_id", 10]]

Create Does Not Work

# artist_tracks.artistic_role_id is totally missing from the INSERT

2.0.0p195 :005 > Track.create!(name: "test_name", lyrics: "test_lyrics", owning_artist_ids: [1])

  Artist Load (1.3ms)  SELECT "artists".* FROM "artists" WHERE "artists"."id" = $1 LIMIT 1  [["id", 1]]
   (0.5ms)  BEGIN
  Artist Exists (0.7ms)  SELECT 1 AS one FROM "artists" WHERE ("artists"."name" = 'TestArtist1' AND "artists"."id" != 1) LIMIT 1
  SQL (0.7ms)  INSERT INTO "tracks" ("created_at", "lyrics", "name", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Thu, 13 Jun 2013 22:20:14 UTC +00:00], ["lyrics", "test_lyrics"], ["name", "test_name"], ["updated_at", Thu, 13 Jun 2013 22:20:14 UTC +00:00]]
#
# Y U NO have artist_tracks.artistic_role_id?
#
  SQL (0.7ms)  INSERT INTO "artist_tracks" ("artist_id", "created_at", "track_id", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["artist_id", 1], ["created_at", Thu, 13 Jun 2013 22:20:14 UTC +00:00], ["track_id", 12], ["updated_at", Thu, 13 Jun 2013 22:20:14 UTC +00:00]]
   (1.0ms)  COMMIT

According to the Rails Guide for Active Record Associations (4.3.3.1 where), I believe my usage of the scope and expectation are valid:

If you use a hash-style where option, then record creation via this association will be automatically scoped using the hash.

Why is the artist_tracks.artistic_role_id attribute being lost? If my expectations are wrong, I'd like to understand why and how to implement an alternative solution.

I have also listed this as an issue on the Rails repo. Any insight is appreciated! Thank you

回答1:

I believe that what is happening is that the associated model actually being created here is the join model, artist_tracks, and not the association with the actual conditions on it. You could probably fix this by declaring an alternate join association with conditions on it, and then attaching owning_artists through that instead. Like this:

class Track < ActiveRecord::Base
  has_many :artist_tracks
  has_many :owning_artist_tracks,
              -> { where(:artistic_role_id => 1) },
              :class_name => "ArtistTrack"
  has_many :owning_artists,
              :through => :owning_artist_tracks,
              :source => :artist
end

来源：https://stackoverflow.com/questions/17098708/losing-an-attribute-when-saving-through-an-association-w-scope-rails-4-0-0