Improve performance of the latest_of_each method of the manager

AUTHORS.rst

@@ -19,6 +19,7 @@ Authors
 - Anton Kulikov (`bigtimecriminal <https://github.com/bigtimecriminal>`_)
 - Ben Lawson (`blawson <https://github.com/blawson>`_)
 - Benjamin Mampaey (`bmampaey <https://github.com/bmampaey>`_)
+- Berke Agababaoglu (`bagababaoglu <https://github.com/bagababaoglu>`_)
 - Bheesham Persaud (`bheesham <https://github.com/bheesham>`_)
 - `bradford281 <https://github.com/bradford281>`_
 - Brian Armstrong (`barm <https://github.com/barm>`_)

CHANGES.rst

@@ -6,6 +6,7 @@ Unreleased
 
 - Made ``skip_history_when_saving`` work when creating an object - not just when
   updating an object (gh-1262)
+- Improved performance of the ``latest_of_each()`` history manager method (gh-1360)
 
 3.7.0 (2024-05-29)
 ------------------

simple_history/manager.py

@@ -1,6 +1,6 @@
 from django.conf import settings
-from django.db import connection, models
-from django.db.models import OuterRef, QuerySet, Subquery
+from django.db import models
+from django.db.models import Exists, OuterRef, Q, QuerySet
 from django.utils import timezone
 
 from simple_history.utils import (
@@ -29,13 +29,11 @@ def __init__(self, *args, **kwargs):
         self._as_of = None
         self._pk_attr = self.model.instance_type._meta.pk.attname
 
-    def as_instances(self):
+    def as_instances(self) -> "HistoricalQuerySet":
         """
         Return a queryset that generates instances instead of historical records.
         Queries against the resulting queryset will translate `pk` into the
         primary key field of the original type.
-
-        Returns a queryset.
         """
         if not self._as_instances:
             result = self.exclude(history_type="-")
@@ -44,7 +42,7 @@ def as_instances(self):
             result = self._clone()
         return result
 
-    def filter(self, *args, **kwargs):
+    def filter(self, *args, **kwargs) -> "HistoricalQuerySet":
         """
         If a `pk` filter arrives and the queryset is returning instances
         then the caller actually wants to filter based on the original
@@ -55,43 +53,26 @@ def filter(self, *args, **kwargs):
             kwargs[self._pk_attr] = kwargs.pop("pk")
         return super().filter(*args, **kwargs)
 
-    def latest_of_each(self):
+    def latest_of_each(self) -> "HistoricalQuerySet":
         """
         Ensures results in the queryset are the latest historical record for each
-        primary key.  Deletions are not removed.
-
-        Returns a queryset.
+        primary key. This includes deletion records.
         """
-        # If using MySQL, need to get a list of IDs in memory and then use them for the
-        # second query.
-        # Does mean two loops through the DB to get the full set, but still a speed
-        # improvement.
-        backend = connection.vendor
-        if backend == "mysql":
-            history_ids = {}
-            for item in self.order_by("-history_date", "-pk"):
-                if getattr(item, self._pk_attr) not in history_ids:
-                    history_ids[getattr(item, self._pk_attr)] = item.pk
-            latest_historics = self.filter(history_id__in=history_ids.values())
-        elif backend == "postgresql":
-            latest_pk_attr_historic_ids = (
-                self.order_by(self._pk_attr, "-history_date", "-pk")
-                .distinct(self._pk_attr)
-                .values_list("pk", flat=True)
-            )
-            latest_historics = self.filter(history_id__in=latest_pk_attr_historic_ids)
-        else:
-            latest_pk_attr_historic_ids = (
-                self.filter(**{self._pk_attr: OuterRef(self._pk_attr)})
-                .order_by("-history_date", "-pk")
-                .values("pk")[:1]
-            )
-            latest_historics = self.filter(
-                history_id__in=Subquery(latest_pk_attr_historic_ids)
-            )
-        return latest_historics
+        # Subquery for finding the records that belong to the same history-tracked
+        # object as the record from the outer query (identified by `_pk_attr`),
+        # and that have a later `history_date` than the outer record.
+        # The very latest record of a history-tracked object should be excluded from
+        # this query - which will make it included in the `~Exists` query below.
+        later_records = self.filter(
+            Q(**{self._pk_attr: OuterRef(self._pk_attr)}),
+            Q(history_date__gt=OuterRef("history_date")),
+        )
+
+        # Filter the records to only include those for which the `later_records`
+        # subquery does not return any results.
+        return self.filter(~Exists(later_records))
 
-    def _select_related_history_tracked_objs(self):
+    def _select_related_history_tracked_objs(self) -> "HistoricalQuerySet":
         """
         A convenience method that calls ``select_related()`` with all the names of
         the model's history-tracked ``ForeignKey`` fields.
@@ -103,18 +84,18 @@ def _select_related_history_tracked_objs(self):
         ]
         return self.select_related(*field_names)
 
-    def _clone(self):
+    def _clone(self) -> "HistoricalQuerySet":
         c = super()._clone()
         c._as_instances = self._as_instances
         c._as_of = self._as_of
         c._pk_attr = self._pk_attr
         return c
 
-    def _fetch_all(self):
+    def _fetch_all(self) -> None:
         super()._fetch_all()
         self._instanceize()
 
-    def _instanceize(self):
+    def _instanceize(self) -> None:
         """
         Convert the result cache to instances if possible and it has not already been
         done.  If a query extracts `.values(...)` then the result cache will not contain

simple_history/tests/tests/test_manager.py

@@ -8,11 +8,66 @@
 from simple_history.manager import SIMPLE_HISTORY_REVERSE_ATTR_NAME
 
 from ..models import Choice, Document, Poll, RankedDocument
+from .utils import HistoricalTestCase
 
 User = get_user_model()
 
 
-class AsOfTest(TestCase):
+class LatestOfEachTestCase(HistoricalTestCase):
+    def test_filtered_instances_are_as_expected(self):
+        document1 = RankedDocument.objects.create(rank=10)
+        document2 = RankedDocument.objects.create(rank=20)
+        document2.rank = 21
+        document2.save()
+        document3 = RankedDocument.objects.create(rank=30)
+        document3.rank = 31
+        document3.save()
+        document3.delete()
+        document4 = RankedDocument.objects.create(rank=40)
+        document4_pk = document4.pk
+        document4.delete()
+        reincarnated_document4 = RankedDocument.objects.create(pk=document4_pk, rank=42)
+
+        record4, record3, record2, record1 = RankedDocument.history.latest_of_each()
+        self.assertRecordValues(
+            record1,
+            RankedDocument,
+            {
+                "rank": 10,
+                "id": document1.id,
+                "history_type": "+",
+            },
+        )
+        self.assertRecordValues(
+            record2,
+            RankedDocument,
+            {
+                "rank": 21,
+                "id": document2.id,
+                "history_type": "~",
+            },
+        )
+        self.assertRecordValues(
+            record3,
+            RankedDocument,
+            {
+                "rank": 31,
+                "id": document3.id,
+                "history_type": "-",
+            },
+        )
+        self.assertRecordValues(
+            record4,
+            RankedDocument,
+            {
+                "rank": 42,
+                "id": reincarnated_document4.id,
+                "history_type": "+",
+            },
+        )
+
+
+class AsOfTestCase(TestCase):
     model = Document
 
     def setUp(self):
@@ -69,7 +124,7 @@ def test_modified(self):
         self.assertEqual(as_of_list[0].changed_by, self.obj.changed_by)
 
 
-class AsOfAdditionalTestCase(TestCase):
+class AsOfTestCaseWithoutSetUp(TestCase):
     def test_create_and_delete(self):
         document = Document.objects.create()
         now = datetime.now()
@@ -150,42 +205,57 @@ def test_historical_query_set(self):
         """
         Demonstrates how the HistoricalQuerySet works to provide as_of functionality.
         """
-        document1 = RankedDocument.objects.create(rank=42)
-        document2 = RankedDocument.objects.create(rank=84)
-        document2.rank = 51
+        document1 = RankedDocument.objects.create(rank=10)
+        document2 = RankedDocument.objects.create(rank=20)
+        document2.rank = 21
         document2.save()
         document1.delete()
+        t1 = datetime.now()
+        document3 = RankedDocument.objects.create(rank=30)  # noqa: F841
+        document2.rank = 22
+        document2.save()
         t2 = datetime.now()
 
-        # look for historical records, get back a queryset
-        with self.assertNumQueries(1):
-            queryset = RankedDocument.history.filter(history_date__lte=t2)
-            self.assertEqual(queryset.count(), 4)
+        # 4 records before `t1` (for document 1 and 2), 2 after (for document 2 and 3)
+        queryset = RankedDocument.history.filter(history_date__lte=t1)
+        self.assertEqual(queryset.count(), 4)
+        self.assertEqual(RankedDocument.history.filter(history_date__gt=t1).count(), 2)
 
-        # only want the most recend records (provided by HistoricalQuerySet)
-        self.assertEqual(queryset.latest_of_each().count(), 2)
+        # `latest_of_each()` returns the most recent record of each document
+        with self.assertNumQueries(1):
+            self.assertEqual(queryset.latest_of_each().count(), 2)
 
-        # want to see the instances as of that time?
-        self.assertEqual(queryset.latest_of_each().as_instances().count(), 1)
+        # `as_instances()` returns the historical instances as of each record's time,
+        # but excludes deletion records (i.e. document 1's most recent record)
+        with self.assertNumQueries(1):
+            self.assertEqual(queryset.latest_of_each().as_instances().count(), 1)
 
-        # these new methods are idempotent
-        self.assertEqual(
-            queryset.latest_of_each()
-            .latest_of_each()
-            .as_instances()
-            .as_instances()
-            .count(),
-            1,
-        )
+        # (Duplicate calls to these methods should not change the number of queries,
+        # since they're idempotent)
+        with self.assertNumQueries(1):
+            self.assertEqual(
+                queryset.latest_of_each()
+                .latest_of_each()
+                .as_instances()
+                .as_instances()
+                .count(),
+                1,
+            )
 
-        # that was all the same as calling as_of!
-        self.assertEqual(
+        self.assertSetEqual(
+            # In conclusion, all of these methods combined...
             set(
-                RankedDocument.history.filter(history_date__lte=t2)
+                RankedDocument.history.filter(history_date__lte=t1)
                 .latest_of_each()
                 .as_instances()
             ),
-            set(RankedDocument.history.as_of(t2)),
+            # ...are equivalent to calling `as_of()`!
+            set(RankedDocument.history.as_of(t1)),
+        )
+
+        self.assertEqual(RankedDocument.history.as_of(t1).get().rank, 21)
+        self.assertListEqual(
+            [d.rank for d in RankedDocument.history.as_of(t2)], [22, 30]
         )
 
 

simple_history/tests/tests/test_models.py

@@ -30,11 +30,6 @@
     pre_create_historical_m2m_records,
     pre_create_historical_record,
 )
-from simple_history.tests.tests.utils import (
-    database_router_override_settings,
-    database_router_override_settings_history_in_diff_db,
-    middleware_override_settings,
-)
 from simple_history.utils import get_history_model_for_model, update_change_reason
 
 from ..external.models import (
@@ -126,6 +121,12 @@
     UUIDModel,
     WaterLevel,
 )
+from .utils import (
+    HistoricalTestCase,
+    database_router_override_settings,
+    database_router_override_settings_history_in_diff_db,
+    middleware_override_settings,
+)
 
 get_model = apps.get_model
 User = get_user_model()
@@ -140,18 +141,10 @@ def get_fake_file(filename):
     return fake_file
 
 
-class HistoricalRecordsTest(TestCase):
+class HistoricalRecordsTest(HistoricalTestCase):
     def assertDatetimesEqual(self, time1, time2):
         self.assertAlmostEqual(time1, time2, delta=timedelta(seconds=2))
 
-    def assertRecordValues(self, record, klass, values_dict):
-        for key, value in values_dict.items():
-            self.assertEqual(getattr(record, key), value)
-        self.assertEqual(record.history_object.__class__, klass)
-        for key, value in values_dict.items():
-            if key not in ["history_type", "history_change_reason"]:
-                self.assertEqual(getattr(record.history_object, key), value)
-
     def test_create(self):
         p = Poll(question="what's up?", pub_date=today)
         p.save()