在Rails has_many中保存关联记录的顺序:通过关联

Question

我正在开发一个Rails插件,其中包含一种修改has_many中关联记录顺序的方法:通过关联.假设我们有以下型号:

class Playlist < ActiveRecord::Base
  has_many :playlists_songs, :dependent => :destroy
  has_many :songs, :through => :playlists_songs
end

class Song < ActiveRecord::Base
  has_many :playlists_songs, :dependent => :destroy
  has_many :playlists, :through => :playlists_songs
end

class PlaylistsSong < ActiveRecord::Base
  belongs_to :playlist
  belongs_to :song
end

如果我们改变播放列表歌曲的顺序(例如@playlist.songs.rotate!),Rails不会触及playlists_songs表中的记录(我正在使用Rails 3.1),这是有道理的.我想调用播放列表的歌曲=方法保存歌曲的顺序,但是,可以通过删除播放列表中的相关现有行并以正确的顺序创建新的行(这样:order => "id"可以在检索它们时使用)或者通过向playlists_songs添加sort:integer列并相应地更新这些值.

我没有看到任何允许这样做的回调(例如before_add).在ActiveRecord :: Associations :: CollectionAssociation中,相关的方法似乎是writer,replace和replace_records,但我对最好的下一步将会失败.有没有办法扩展或安全地覆盖这些方法之一,以允许我正在寻找的功能(最好只针对特定的关联),或者是否有一个不同的,更好的方法呢？

Answer 1

你看过acts_as_list吗？它是最老派的rails插件之一,旨在解决这类问题.

它不是排序id,而是对位置列进行排序.然后,这只是更新位置的问题,而不是更改id或删除/替换记录的混乱业务.

在您的情况下,您只需添加一个position整数列PlayListSong,然后:

class PlayListSong
  acts_as_list :scope => :play_list_id
end

正如您在评论中指出的那样,这些方法acts_as_list主要针对列表中的各个项目,并且没有开箱即用的"重新排序"功能.我不建议篡改replace_records这样做.编写一个使用与插件相同的位置列的方法会更清晰,更明确.例如.

class PlayList
  # It makes sense for these methods to be on the association.  You might make it
  # work for #songs instead (as in your question), but the join table is what's
  # keeping the position.
  has_many :play_list_songs, ... do

    # I'm not sure what rotate! should do, so...

    # This first method makes use of acts_as_list's functionality
    #
    # This should take the last song and move it to the first, incrementing 
    # the position of all other songs, effectively rotating the list forward 
    # by 1 song.
    def rotate!
      last.move_to_top unless empty?
    end

    # this, on the other hand, would reorder given an array of play_list_songs.
    # 
    # Note: this is a rough (untested) idea and could/should be reworked for 
    # efficiency and safety. 
    def reorder!(reordered_songs)
      position = 0
      reordered_songs.each do |song|
        position += 1

        # Note: update_column is 3.1+, but I'm assuming you're using it, since
        # that was the source you linked to in your question
        find(song.id).update_column(:position, position)
      end
    end
  end
end