Saving the order of associated records in a Rails has_many :through association

Question

I'm working on a Rails plugin that includes a way to modify the order of associated records in a has_many :through association. Say we have the following models:

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

If we change the order of a Playlist's Songs (e.g. @playlist.songs.rotate!), Rails doesn't touch the records in the playlists_songs table (I'm using Rails 3.1), which makes sense. I'd like to make any call to Playlist's songs= method save the order of the Songs, though, perhaps by either deleting the relevant existing rows in playlists_songs and creating new ones in the proper order (so that :order => "id" could be used when retrieving them) or by adding a sort:integer column to playlists_songs and updating those values accordingly.

I didn't see any callbacks (e.g. before_add) that would allow this. In ActiveRecord::Associations::CollectionAssociation, the relevant methods seem to be writer, replace, and replace_records, but I'm lost on what the best next step would be. Is there a way to extend or safely override one of these methods to allow for the functionality I'm seeking (preferably for only specific associations), or is there a different, better approach for this?

Answer 1

Have you looked at acts_as_list? It's one of the most old-school of rails plugins, and is intended to handle this sort of problem.

Rather than sorting on id, it sorts on a positional column. Then it's simply a matter of updating the position, rather than the messy business of changing the id or deleting/replacing records.

In your case, you'd simply add a position integer column to PlayListSong, then:

class PlayListSong
  acts_as_list :scope => :play_list_id
end

As you point out in the comments, the methods in acts_as_list work mostly on individual items in the list, and there's no "reorder" functionality out of the box. I would not recommend tampering with replace_records to do this. It would be cleaner and more explicit to write a method making use of the same position column as the plugin. For example.

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