Add support for GenericRelation field

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added support for Django's reverse generic relations (`GenericRelation` model field) ([#93](https://github.com/dabapps/django-readers/pull/93)).
+
 ## [2.1.2] - 2023-07-17
 
 ### Fixed

django_readers/qs.py

@@ -1,3 +1,4 @@
+from django.contrib.contenttypes.fields import ReverseGenericManyToOneDescriptor
 from django.db.models import Prefetch, QuerySet
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.fields.related_descriptors import (
@@ -167,6 +168,37 @@ def prefetch_reverse_relationship(
     )
 
 
+def prefetch_reverse_generic_relationship(
+    name,
+    content_type_field_name,
+    object_id_field_name,
+    related_queryset,
+    prepare_related_queryset=noop,
+    to_attr=None,
+):
+    """
+    Efficiently prefetch a reverse generic relationship: one where the field on the "parent"
+    queryset is a `GenericRelation` field. We need to include this field in the query.
+    """
+    return pipe(
+        include_fields(name),
+        prefetch_related(
+            Prefetch(
+                name,
+                pipe(
+                    include_fields(
+                        "pk",
+                        content_type_field_name,
+                        object_id_field_name,
+                    ),
+                    prepare_related_queryset,
+                )(related_queryset),
+                to_attr,
+            )
+        ),
+    )
+
+
 def prefetch_many_to_many_relationship(
     name, related_queryset, prepare_related_queryset=noop, to_attr=None
 ):
@@ -246,5 +278,14 @@ def prepare(queryset):
                 prepare_related_queryset,
                 to_attr,
             )(queryset)
+        if type(related_descriptor) is ReverseGenericManyToOneDescriptor:
+            return prefetch_reverse_generic_relationship(
+                name,
+                related_descriptor.rel.field.content_type_field_name,
+                related_descriptor.rel.field.object_id_field_name,
+                related_descriptor.field.related_model.objects.all(),
+                prepare_related_queryset,
+                to_attr,
+            )(queryset)
 
     return prepare

django_readers/rest_framework.py

@@ -1,4 +1,5 @@
 from copy import deepcopy
+from django.contrib.contenttypes.fields import ReverseGenericManyToOneDescriptor
 from django.core.exceptions import ImproperlyConfigured
 from django.utils.functional import cached_property
 from django_readers import specs
@@ -124,10 +125,25 @@ def _get_child_serializer_kwargs(self, rel_info):
             kwargs["allow_null"] = True
         return kwargs
 
+    def _get_rel_info(self, rel_name):
+        descriptor = getattr(self.model, rel_name)
+        # Special case for reverse generic relations (GenericRelation field)
+        # as these don't appear in rest-framework's rel_info
+        if isinstance(descriptor, ReverseGenericManyToOneDescriptor):
+            return model_meta.RelationInfo(
+                model_field=descriptor.field,
+                related_model=descriptor.field.related_model,
+                to_many=True,
+                to_field=None,
+                has_through_model=False,
+                reverse=True,
+            )
+        return self.info.relations[rel_name]
+
     def visit_dict_item_list(self, key, value):
         # This is a relationship, so we recurse and create
         # a nested serializer to represent it
-        rel_info = self.info.relations[key]
+        rel_info = self._get_rel_info(key)
         capfirst = self._lowercase_with_underscores_to_capitalized_words(key)
         child_serializer_class = serializer_class_for_spec(
             f"{self.name}{capfirst}",
@@ -143,7 +159,7 @@ def visit_dict_item_dict(self, key, value):
         # do the same as the previous case, but handled
         # slightly differently to set the `source` correctly
         relationship_name, relationship_spec = next(iter(value.items()))
-        rel_info = self.info.relations[relationship_name]
+        rel_info = self._get_rel_info(relationship_name)
         capfirst = self._lowercase_with_underscores_to_capitalized_words(key)
         child_serializer_class = serializer_class_for_spec(
             f"{self.name}{capfirst}",

tests/models.py

@@ -1,6 +1,17 @@
+from django.contrib.contenttypes.fields import GenericRelation
 from django.db import models
 
 
+class LogEntry(models.Model):
+    content_type = models.ForeignKey(
+        to="contenttypes.ContentType",
+        on_delete=models.CASCADE,
+        related_name="+",
+    )
+    object_pk = models.CharField(max_length=255)
+    event = models.CharField(max_length=100)
+
+
 class Group(models.Model):
     name = models.CharField(max_length=100)
 
@@ -15,6 +26,9 @@ class Widget(models.Model):
     value = models.PositiveIntegerField(default=0)
     other = models.CharField(max_length=100, null=True)
     owner = models.ForeignKey(Owner, null=True, on_delete=models.SET_NULL)
+    logs = GenericRelation(
+        LogEntry, content_type_field="content_type", object_id_field="object_pk"
+    )
 
 
 class Thing(models.Model):

tests/test_qs.py

@@ -1,9 +1,10 @@
+from django.contrib.contenttypes.models import ContentType
 from django.db import connection
 from django.db.models import Count
 from django.test import TestCase
 from django.test.utils import CaptureQueriesContext
 from django_readers import qs
-from tests.models import Category, Owner, Widget
+from tests.models import Category, LogEntry, Owner, Widget
 from unittest import mock
 
 
@@ -188,6 +189,84 @@ def test_prefetch_reverse_relationship(self):
         with self.assertNumQueries(0):
             self.assertEqual(owners[0].widget_set.all()[0].name, "test widget")
 
+    def test_prefetch_reverse_generic_relationship(self):
+        widget = Widget.objects.create(name="test widget")
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="CREATED",
+        )
+
+        prepare = qs.pipe(
+            qs.include_fields("name"),
+            qs.prefetch_reverse_generic_relationship(
+                "logs",
+                "content_type",
+                "object_pk",
+                LogEntry.objects.all(),
+                qs.include_fields("event"),
+            ),
+        )
+
+        with CaptureQueriesContext(connection) as capture:
+            widgets = list(prepare(Widget.objects.all()))
+
+        self.assertEqual(len(capture.captured_queries), 2)
+
+        self.assertEqual(
+            capture.captured_queries[0]["sql"],
+            "SELECT "
+            '"tests_widget"."id", '
+            '"tests_widget"."name" '
+            "FROM "
+            '"tests_widget"',
+        )
+
+        content_type_id = ContentType.objects.get_for_model(Widget).pk
+
+        self.assertEqual(
+            capture.captured_queries[1]["sql"],
+            "SELECT "
+            '"tests_logentry"."id", '
+            '"tests_logentry"."content_type_id", '
+            '"tests_logentry"."object_pk", '
+            '"tests_logentry"."event" '
+            "FROM "
+            '"tests_logentry" '
+            "WHERE "
+            f'("tests_logentry"."content_type_id" = {content_type_id} AND '
+            '"tests_logentry"."object_pk" IN '
+            "('1'))",
+        )
+
+        with self.assertNumQueries(0):
+            self.assertEqual(widgets[0].logs.all()[0].event, "CREATED")
+
+    def test_prefetch_reverse_generic_relationship_with_to_attr(self):
+        widget = Widget.objects.create(name="test widget")
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="CREATED",
+        )
+
+        prepare = qs.pipe(
+            qs.include_fields("name"),
+            qs.prefetch_reverse_generic_relationship(
+                "logs",
+                "content_type",
+                "object_pk",
+                LogEntry.objects.all(),
+                qs.include_fields("event"),
+                to_attr="history",
+            ),
+        )
+
+        widgets = list(prepare(Widget.objects.all()))
+
+        with self.assertNumQueries(0):
+            self.assertEqual(widgets[0].history[0].event, "CREATED")
+
     def test_prefetch_reverse_relationship_only_loads_pk_and_related_name_by_default(
         self,
     ):
@@ -358,6 +437,12 @@ def test_auto_prefetch_relationship(self):
             qs.auto_prefetch_relationship("category_set")(Widget.objects.all())
             mock_fn.assert_called_once()
 
+        with mock.patch(
+            "django_readers.qs.prefetch_reverse_generic_relationship"
+        ) as mock_fn:
+            qs.auto_prefetch_relationship("logs")(Widget.objects.all())
+            mock_fn.assert_called_once()
+
     def test_annotate_only_includes_fk_by_default(self):
         owner = Owner.objects.create(name="test owner")
         Widget.objects.create(name="test 1", owner=owner)

tests/test_rest_framework.py

@@ -1,3 +1,4 @@
+from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase
 from django_readers import pairs, qs
@@ -10,7 +11,7 @@
 from rest_framework import serializers
 from rest_framework.generics import ListAPIView, RetrieveAPIView
 from rest_framework.test import APIRequestFactory
-from tests.models import Category, Group, Owner, Widget
+from tests.models import Category, Group, LogEntry, Owner, Widget
 from textwrap import dedent
 
 
@@ -28,6 +29,7 @@ class WidgetListView(SpecMixin, ListAPIView):
                 },
             ]
         },
+        {"logs": ["event"]},
     ]
 
 
@@ -53,17 +55,32 @@ class CategoryDetailView(SpecMixin, RetrieveAPIView):
 
 class RESTFrameworkTestCase(TestCase):
     def test_list(self):
-        Widget.objects.create(
+        widget = Widget.objects.create(
             name="test widget",
             owner=Owner.objects.create(
                 name="test owner", group=Group.objects.create(name="test group")
             ),
         )
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="CREATED",
+        )
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="UPDATED",
+        )
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="DELETED",
+        )
 
         request = APIRequestFactory().get("/")
         view = WidgetListView.as_view()
 
-        with self.assertNumQueries(3):
+        with self.assertNumQueries(4):
             response = view(request)
 
         self.assertEqual(
@@ -77,6 +94,11 @@ def test_list(self):
                             "name": "test group",
                         },
                     },
+                    "logs": [
+                        {"event": "CREATED"},
+                        {"event": "UPDATED"},
+                        {"event": "DELETED"},
+                    ],
                 }
             ],
         )
@@ -180,12 +202,16 @@ def test_all_relationship_types(self):
                             },
                         ]
                     },
+                    {
+                        "logs": [
+                            "event",
+                        ]
+                    },
                 ]
             },
         ]
 
         cls = serializer_class_for_spec("Owner", Owner, spec)
-
         expected = dedent(
             """\
             OwnerSerializer():
@@ -199,7 +225,9 @@ def test_all_relationship_types(self):
                     thing = OwnerWidgetSetThingSerializer(read_only=True):
                         name = CharField(max_length=100, read_only=True)
                         related_widget = OwnerWidgetSetThingRelatedWidgetSerializer(allow_null=True, read_only=True, source='widget'):
-                            name = CharField(allow_null=True, max_length=100, read_only=True, required=False)"""
+                            name = CharField(allow_null=True, max_length=100, read_only=True, required=False)
+                    logs = OwnerWidgetSetLogsSerializer(allow_null=True, many=True, read_only=True):
+                        event = CharField(max_length=100, read_only=True)"""
         )
         self.assertEqual(repr(cls()), expected)
 

tests/test_specs.py

@@ -1,6 +1,7 @@
+from django.contrib.contenttypes.models import ContentType
 from django.test import TestCase
 from django_readers import specs
-from tests.models import Category, Group, Owner, Thing, Widget
+from tests.models import Category, Group, LogEntry, Owner, Thing, Widget
 
 
 class SpecTestCase(TestCase):
@@ -28,20 +29,26 @@ def test_relationships(self):
         category = Category.objects.create(name="test category")
         category.widget_set.add(widget)
         Thing.objects.create(name="test thing", widget=widget)
+        LogEntry.objects.create(
+            content_type=ContentType.objects.get_for_model(widget),
+            object_pk=widget.id,
+            event="CREATED",
+        )
 
         prepare, project = specs.process(
             [
                 "name",
                 {"owner": ["name", {"widget_set": ["name"]}]},
                 {"category_set": ["name", {"widget_set": ["name"]}]},
                 {"thing": ["name", {"widget": ["name"]}]},
+                {"logs": ["event"]},
             ]
         )
 
         with self.assertNumQueries(0):
             queryset = prepare(Widget.objects.all())
 
-        with self.assertNumQueries(7):
+        with self.assertNumQueries(8):
             instance = queryset.first()
 
         with self.assertNumQueries(0):
@@ -59,6 +66,7 @@ def test_relationships(self):
                     {"name": "test category", "widget_set": [{"name": "test widget"}]},
                 ],
                 "thing": {"name": "test thing", "widget": {"name": "test widget"}},
+                "logs": [{"event": "CREATED"}],
             },
         )