[longplay] Fix recompute script

PROJECT.org

@@ -88,7 +88,7 @@ fetching and simple saving.
 *** Metadata sources
 **** Scraper
 
-* Backlog [0/20] :vrobbler:project:personal:
+* Backlog [1/21] :vrobbler:project:personal:
 ** TODO [#C] Create small utility to clean up tracks scrobbled with wonky playback times :bug:music:scrobbles:
 :PROPERTIES:
 :ID:       702462cf-d54b-48c6-8a7c-78b8de751deb
@@ -594,6 +594,10 @@ named constants for maintainability.
   - ~vrobbler/apps/webpages/models.py~ (line 290) -- ="url"=
   - ~vrobbler/apps/scrobbles/importers/tsv.py~ (line 55) -- ="S"= completion status
 
+** DONE [#A] Fix bug in recomputing long play seconds taking forever :bug:longplay:commands:
+:PROPERTIES:
+:ID:       0a813cf9-17fb-dbd7-b5a7-7410d9bd4d8c
+:END:
 * Version 52.1 [1/1]
 ** DONE [#C] Show time per scrobble in long play lists and total time playing :templates:longplay:scrobbles:
 :PROPERTIES:

vrobbler/apps/scrobbles/management/commands/recompute_long_play_seconds.py

@@ -4,11 +4,14 @@ from django.db import connection
 from scrobbles.constants import LONG_PLAY_MEDIA
 from scrobbles.models import Scrobble
 
+BATCH_SIZE = 1000
+
 
 class Command(BaseCommand):
     help = (
         "Backfill long_play_last_scrobble FK chains, then recompute "
-        "long_play_seconds by walking backward through the chain."
+        "long_play_seconds by walking forward through scrobbles in "
+        "timestamp order with a running accumulator."
     )
 
     def add_arguments(self, parser):
@@ -61,49 +64,74 @@ class Command(BaseCommand):
 
         # Step 2: recompute long_play_seconds
         self.stdout.write(
-            "\nStep 2: Recomputing long_play_seconds via FK chain..."
+            "\nStep 2: Recomputing long_play_seconds in timestamp order..."
         )
 
-        scrobbles = Scrobble.objects.filter(
-            media_type__in=media_types,
-            playback_position_seconds__isnull=False,
-        ).order_by("-timestamp")
-
-        total = scrobbles.count()
-        self.stdout.write(f"  Found {total} long play scrobbles to process")
-
-        to_update = []
-        for scrobble in scrobbles.iterator():
-            accumulated = scrobble.playback_position_seconds or 0
-            current = scrobble.long_play_last_scrobble
-            while current and not current.long_play_complete:
-                accumulated += current.playback_position_seconds or 0
-                current = current.long_play_last_scrobble
-
-            if scrobble.long_play_seconds != accumulated:
-                self.stdout.write(
-                    f"  Scrobble {scrobble.id} ({scrobble.media_type}): "
-                    f"{scrobble.long_play_seconds or 0} -> {accumulated}"
-                )
-                if not dry_run:
-                    scrobble.long_play_seconds = accumulated
-                    to_update.append(scrobble)
-
-        if to_update:
-            Scrobble.objects.bulk_update(to_update, ["long_play_seconds"])
+        total_updated = 0
+        for mt in media_types:
+            n = self._recompute_for_media_type(mt, dry_run)
+            total_updated += n
+            self.stdout.write(f"  {mt}: {n} scrobbles updated")
 
         if dry_run:
             self.stdout.write(
                 self.style.WARNING(
-                    f"Dry run: would update {len(to_update)} scrobbles. "
+                    f"Dry run: would update {total_updated} scrobbles total. "
                     "Use without --dry-run to apply."
                 )
             )
         else:
             self.stdout.write(
-                self.style.SUCCESS(f"Updated {len(to_update)} scrobbles")
+                self.style.SUCCESS(f"Updated {total_updated} scrobbles")
             )
 
+    def _recompute_for_media_type(self, media_type: str, dry_run: bool) -> int:
+        """Process scrobbles for a single media type in timestamp order with a
+        running accumulator, avoiding O(n2) FK chain walks."""
+        fk = _media_type_to_fk(media_type)
+        fk_id = f"{fk}_id"
+
+        scrobbles = Scrobble.objects.filter(
+            media_type=media_type,
+            **{f"{fk}__isnull": False},
+            playback_position_seconds__isnull=False,
+        ).order_by(fk_id, "user_id", "timestamp")
+
+        total = scrobbles.count()
+        if not total:
+            return 0
+
+        updated = 0
+        batch = []
+        last_key = None
+        running_total = 0
+
+        for scrobble in scrobbles.iterator():
+            key = (getattr(scrobble, fk_id), scrobble.user_id)
+
+            if key != last_key:
+                running_total = 0
+                last_key = key
+
+            running_total += scrobble.playback_position_seconds or 0
+
+            if scrobble.long_play_seconds != running_total:
+                updated += 1
+                if not dry_run:
+                    scrobble.long_play_seconds = running_total
+                    batch.append(scrobble)
+                    if len(batch) >= BATCH_SIZE:
+                        Scrobble.objects.bulk_update(batch, ["long_play_seconds"])
+                        batch = []
+
+            if scrobble.long_play_complete:
+                running_total = 0
+
+        if batch:
+            Scrobble.objects.bulk_update(batch, ["long_play_seconds"])
+
+        return updated
+
     def _backfill_chain(self, media_type: str, dry_run: bool) -> int:
         """Set long_play_last_scrobble on each scrobble to the previous
         scrobble for the same media+user using a single UPDATE with a