New "index" field on playlist tracks, and .insert() metod to manage playlists

api/funkwhale_api/playlists/models.py

@@ -1,4 +1,6 @@
+from django import forms
 from django.db import models
+from django.db import transaction
 from django.utils import timezone
 
 from funkwhale_api.common import fields
@@ -14,14 +16,59 @@ class Playlist(models.Model):
     def __str__(self):
         return self.name
 
-    def add_track(self, track, previous=None):
-        plt = PlaylistTrack(previous=previous, track=track, playlist=self)
-        plt.save()
+    @transaction.atomic
+    def insert(self, plt, index=None):
+        """
+        Given a PlaylistTrack, insert it at the correct index in the playlist,
+        and update other tracks index if necessary.
+        """
+        old_index = plt.index
+        move = old_index is not None
+        if index is not None and index == old_index:
+            # moving at same position, just skip
+            return index
 
-        return plt
+        existing = self.playlist_tracks.select_for_update()
+        if move:
+            existing = existing.exclude(pk=plt.pk)
+        total = existing.filter(index__isnull=False).count()
+
+        if index is None:
+            # we simply increment the last track index by 1
+            index = total
+
+        if index > total:
+            raise forms.ValidationError('Index is not continuous')
+
+        if index < 0:
+            raise forms.ValidationError('Index must be zero or positive')
+
+        if move:
+            # we remove the index temporarily, to avoid integrity errors
+            plt.index = None
+            plt.save(update_fields=['index'])
+
+        if move:
+            if index > old_index:
+                # new index is higher than current, we decrement previous tracks
+                to_update = existing.filter(
+                    index__gt=old_index, index__lte=index)
+                to_update.update(index=models.F('index') - 1)
+            if index < old_index:
+                # new index is lower than current, we increment next tracks
+                to_update = existing.filter(
+                    index__lt=old_index, index__gte=index)
+                to_update.update(index=models.F('index') + 1)
+        else:
+            to_update = existing.filter(index__gte=index)
+            to_update.update(index=models.F('index') + 1)
+
+        plt.index = index
+        plt.save(update_fields=['index'])
+        return index
 
 
-class PlaylistTrack(MPTTModel):
+class PlaylistTrack(models.Model):
     track = models.ForeignKey(
         'music.Track',
         related_name='playlist_tracks',
@@ -29,6 +76,7 @@ class PlaylistTrack(MPTTModel):
     index = models.PositiveIntegerField(null=True)
     playlist = models.ForeignKey(
         Playlist, related_name='playlist_tracks', on_delete=models.CASCADE)
+    creation_date = models.DateTimeField(default=timezone.now)
 
     class Meta:
         ordering = ('-playlist', 'index')