Update rankings/priorities at one go in Ruby on Rails - ruby-on-rails

Question

I am working on a personal project in Ruby on Rails in which I have a song model which has a field called priority. A user can add songs to a playlist and set priority to each song, which determines the order in which the songs are played. The problem I am stuck with here, is that I need unique priorities.
So if I have a playlist with 8 songs, and I am adding song number 9 with priority 4, the remaining songs should get their priorities updated. Can any one suggest what is the best way of going about this ?

Answer 1

I'm actually tasked with a similar problem in my own application.
Here's my first take:
class Playlist
has_many :playlist_songs
has_many :songs, through: :playlist_songs
end
--
class PlaylistSong
before_save :update_priorities, if: :priorities_changed?
def priorities_changed?
new_record? || priority_was != priority
end
def update_priorities
if new_record?
playlist.playlist_songs.update_all("priority = priority+1 where priority >= #{priority}")
else
# ouch. not so simple with unique constraint on priority
end
end
end
The problem with the unique constraint is this:
priorities: 1,2,3
update: 3 -> 1
#> triggers
1 -> 2
2 -> 3
But 3 is already taken (the current record in which we're changing priority to 1). We cannot change to 1 first because 1 is taken. The only idea I have at the minute is to set the currently updated record to a non-unique priority like 999, but then we're affectively locking updates on the priorities between that query and updating the priority list. I guess you could temporarily set it to a large random number to avoid such unlikely conflicts.
See here for why this is particularly tricky:
Reordering an ordered list

Answer 2

Shouldn't you be able to use the validates_uniqueness_of constraint in the Song model to achieve this?
class Song < ActiveRecord::Base
validates :priority, numericality: { only_integer: true }, :uniqueness => {:scope => :playlist_id}
end
This should force the user to type in a different number other than the ones already saved for a particular playlist...

Answer 3

This is the way I dealt with it - if I enforced a uniqueness constraint, I will have to go with the way Damien has dealt - by assigning some unique priority and then updating. So instead, I decided to let my code deal with the uniqueness. Here's what I have done :
While creating a new song for the playlist, I am calling the create_new_priorities_for_existing_songs in the songs#create action -
def self.create_new_priorities_for_existing_songs (song, priority)
song_id = song.id
playlist = song.playlist
all_songs = playlist.songs
all_songs.where("priority >= :priority and id != :song_id", :priority => priority, :song_id => song_id).update_all ("priority = priority+1")
end
I am calling the update_priorities_of_existing_songs method in my songs#update action -
def self.update_priorities_of_existing_songs (song_id, old_priority, new_priority)
return if old_priority == new_priority
song = Song.find(song_id)
playlist = song.playlist
all_songs = playlist.songs
if old_priority < new_priority
all_songs.where("priority > :old_priority and priority <= :new_priority and id != :song_id", :old_priority => old_priority, :new_priority => new_priority, :song_id => song_id).update_all ("priority = priority-1")
elsif old_priority > new_priority
all_songs.where("priority >= :new_priority and priority < :old_priority and id != :song_id", :old_priority => old_priority, :new_priority => new_priority, :song_id => song_id).update_all ("priority = priority+1")
end
end