Add allowed_campaign_count field to Channel model and implement counter cache logic

twitch/migrations/0021_channel_allowed_campaign_count_cache.py

@@ -0,0 +1,83 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from django.db import migrations
+from django.db import models
+
+if TYPE_CHECKING:
+    from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+    from django.db.migrations.state import StateApps
+
+    from twitch.models import Channel
+    from twitch.models import DropCampaign
+
+
+def backfill_allowed_campaign_count(
+    apps: StateApps,
+    schema_editor: BaseDatabaseSchemaEditor,
+) -> None:
+    """Populate Channel.allowed_campaign_count from the M2M through table."""
+    del schema_editor
+
+    Channel: type[Channel] = apps.get_model("twitch", "Channel")
+    DropCampaign: type[DropCampaign] = apps.get_model("twitch", "DropCampaign")
+    through_model: type[Channel] = DropCampaign.allow_channels.through
+
+    counts_by_channel = {
+        row["channel_id"]: row["campaign_count"]
+        for row in (
+            through_model.objects.values("channel_id").annotate(
+                campaign_count=models.Count("dropcampaign_id"),
+            )
+        )
+    }
+
+    channels: list[Channel] = list(
+        Channel.objects.all().only("id", "allowed_campaign_count"),
+    )
+    for channel in channels:
+        channel.allowed_campaign_count = counts_by_channel.get(channel.pk, 0)
+
+    if channels:
+        Channel.objects.bulk_update(
+            channels,
+            ["allowed_campaign_count"],
+            batch_size=1000,
+        )
+
+
+def noop_reverse(
+    apps: StateApps,
+    schema_editor: BaseDatabaseSchemaEditor,
+) -> None:
+    """No-op reverse migration for cached counters."""
+    del apps
+    del schema_editor
+
+
+class Migration(migrations.Migration):
+    """Add cached channel campaign counts and backfill existing rows."""
+
+    dependencies = [
+        ("twitch", "0020_rewardcampaign_tw_reward_ends_starts_idx"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="channel",
+            name="allowed_campaign_count",
+            field=models.PositiveIntegerField(
+                default=0,
+                help_text="Cached number of drop campaigns that allow this channel.",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="channel",
+            index=models.Index(
+                fields=["-allowed_campaign_count", "name"],
+                name="tw_chan_cc_name_idx",
+            ),
+        ),
+        migrations.RunPython(backfill_allowed_campaign_count, noop_reverse),
+    ]