Melodia/archiver/playlist.py

from django.db import models

from song import Song
from listfield import IntegerListField

"""
Playlist model
Each playlist is a high-level ordering of songs. There really isn't much to a playlist - just its name, and the songs inside it.
However, we need to have a way to guarantee song order, in addition to re-ordering. A ManyToMany field can't do this.
As such, a custom IntegerListField is implemented - it takes a python list of ints, converts it to a text field in the DB,
and then back to a python list. This way, we can guarantee order, and have a song appear multiple times.
The IntegerListField itself uses the ID of each song as the int in a list. For example, a list of:
	[1, 3, 5, 17]

Means that the playlist is made up of four songs. The order of the playlist is the song with index 1, 3, 5, and 17.
Additionally, the ManyToMany field is included to make sure we don't use the global Songs manager - it just seems hackish.
"""

class Playlist (models.Model):
	"The Playlist class defines the playlist model and its operations"

	name       = models.CharField(max_length = 255)
	song_order = IntegerListField()

	#This is a bit of a backup field, since technically the song PK's are in
	#the song_order field. However, it might be useful to just get a reference
	#to the songs in this playlist. Additionally, it's kind of hackish to reference
	#the global Song manager, rather than something that is specific for this playlist.
	songs      = models.ManyToManyField(Song)

	def _populate_songs(self):
		"""Make sure that the 'songs' relation is up-to-date."""
		#This operation works by getting the ID's for all songs currently in the playlist,
		#calculates what we need to add, what we need to remove, and then does it.
		#As much work as is possible is done on the python side to avoid the DB
		current_song_ids     = [song.id for song in self.songs.all()]
		current_song_ids_set = set(current_song_ids)

		new_song_ids_set = set(song_order)

		remove_set = current_song_ids_set.difference(new_song_ids_set)
		add_set    = new_song_ids_set.difference(current_song_ids_set)

		for song_id in remove_set:
			song = self.songs.get(id = song_id)
			song.remove()

		for song_id in add_set:
			song = Song.objects.get(id = song_id) #Using the global Songs manager is unavoidable for this one
			self.songs.add(song)

	def insert(self, position, new_song):
		"""Insert a new song into the playlist at a specific position."""

		if not isinstance(new_song, Song):
			#Not given a song reference, raise an error
			raise ValidationError("Not given a song reference to insert.")
			
		self.songs.add(new_song)

		self.song_order.insert(position, new_song.id)

		_populate_songs()

	def append(self, new_song):
		"""Add a new song to the end of the playlist."""
		if not isinstance(new_song, Song):
			#Not given a song reference, raise an error
			raise ValidationError("Not given a song reference to insert.")

		self.songs.add(new_song)

		self.song_order.append(new_song.id)

	def move(self, original_position, new_position):
		"""Move a song from one position to another"""
		if original_position == new_position:
			return

		song_id = self.song_order[original_position]

		#This is actually a bit more complicated than it first appears, since the index we're moving to may actually change
		#when we remove an item.
		if new_position < original_position:
			del self.song_order[original_position]
			self.song_order.insert(new_position, song_id)

		else:
			del self.song_order[original_position]
			self.song_order.insert(new_position - 1, song_id) #Account for the list indices shifting down.

	def remove(self, position):
		if position > len(self.song_order):
			return False

		del self.song_order[position]

	def export(self, playlist_type = "m3u"):
		"""Export this internal playlist to a file format."""
		#Default m3u playlist type, support others as I build them.

		if playlist_type == "pls":
			pass

		else:
			#Export m3u, default option
			pass