Channels API: improver performance, remove CRUD (#3388)

Author: mbertrand

channels/api.py

@@ -9,8 +9,5 @@ class ChannelsConfig(AppConfig):
     name = "channels"
 
     def ready(self):
-        """
-        Ready handler. Import signals.
-        """
-        import channels.signals  # noqa: F401
+        """Ready handler."""
         from channels import schema  # noqa: F401

channels/api_test.py

@@ -3,7 +3,6 @@
 import factory
 from factory.django import DjangoModelFactory
 
-from channels.api import create_channel_groups_and_roles
 from channels.constants import ChannelType
 from channels.models import (
     Channel,
@@ -70,19 +69,6 @@ class ChannelFactory(DjangoModelFactory):
         ),
     )
 
-    @factory.post_generation
-    def create_roles(
-        self,
-        create,
-        extracted,  # noqa: ARG002
-        **kwargs,  # noqa: ARG002
-    ):  # pylint: disable=unused-argument
-        """Create the channel groups and roles after the channel is created"""
-        if not create:
-            return
-
-        create_channel_groups_and_roles(self)
-
     class Meta:
         model = Channel
         skip_postgeneration_save = True

channels/apps.py

@@ -5,7 +5,7 @@
 from django.contrib.auth.models import Group
 from django.core.validators import RegexValidator
 from django.db import models
-from django.db.models import Case, JSONField, When, deletion
+from django.db.models import Case, JSONField, Prefetch, When, deletion
 from django.db.models.functions import Concat
 from imagekit.models import ImageSpecField, ProcessedImageField
 from imagekit.processors import ResizeToFit
@@ -47,6 +47,40 @@ def annotate_channel_url(self):
             )
         )
 
+    def with_detail_relations(self) -> "ChannelQuerySet":
+        """
+        Apply the prefetch/select_related/annotate chain needed to serialize
+        a full ChannelSerializer without N+1 queries.
+        """
+        return (
+            self.prefetch_related(
+                Prefetch(
+                    "lists",
+                    queryset=ChannelList.objects.select_related(
+                        "channel_list"
+                    ).order_by("position"),
+                ),
+                Prefetch(
+                    "sub_channels",
+                    queryset=SubChannel.objects.select_related(
+                        "parent_channel"
+                    ).order_by("position"),
+                ),
+                Prefetch(
+                    "sub_channels__channel",
+                    queryset=Channel.objects.annotate_channel_url(),
+                ),
+            )
+            .annotate_channel_url()
+            .select_related(
+                "featured_list",
+                "topic_detail",
+                "department_detail",
+                "unit_detail",
+                "pathway_detail",
+            )
+        )
+
 
 class Channel(TimestampedModel):
     """Channel for any field/subject"""
@@ -132,6 +166,16 @@ def channel_url(self) -> str | None:
             return frontend_absolute_url(f"/c/{self.channel_type}/{self.name}/")
         return None
 
+    @property
+    def ordered_lists(self) -> list[LearningResource]:
+        """The channel's LearningPaths, ordered by position.
+
+        Resolves through the prefetched ``lists`` relation when available
+        (see ``ChannelQuerySet.with_detail_relations``) so serializing a
+        channel does not trigger an extra query per list.
+        """
+        return [channel_list.channel_list for channel_list in self.lists.all()]
+
     class Meta:
         unique_together = ("name", "channel_type")
 

channels/factories.py

@@ -5,9 +5,13 @@
 from channels.constants import ChannelType
 from channels.factories import (
     ChannelDepartmentDetailFactory,
+    ChannelFactory,
     ChannelTopicDetailFactory,
     ChannelUnitDetailFactory,
+    SubChannelFactory,
 )
+from channels.models import Channel
+from channels.serializers import ChannelSerializer
 
 pytestmark = [pytest.mark.django_db]
 
@@ -37,3 +41,19 @@ def test_channel_url_for_departments(published, channel_type, detail_factory):
         )
     else:
         assert channel.channel_url is None
+
+
+@pytest.mark.parametrize("sub_channel_count", [1, 5, 10])
+def test_with_detail_relations_serialization_has_no_extra_subchannel_queries(
+    django_assert_num_queries, sub_channel_count
+):
+    """with_detail_relations should preload subchannel relations for serialization."""
+    channel = ChannelFactory.create(is_topic=True)
+    SubChannelFactory.create_batch(sub_channel_count, parent_channel=channel)
+
+    prefetched_channel = Channel.objects.with_detail_relations().get(pk=channel.pk)
+
+    with django_assert_num_queries(0):
+        serialized = ChannelSerializer(prefetched_channel).data
+
+    assert len(serialized["sub_channels"]) == sub_channel_count