Added _select_related_history_tracked_objs()

...to `HistoricalQuerySet`.
This convenience method will make it easier to avoid database queries
- e.g. when diffing and listing the results, which will be done as part
of the upcoming changes to the admin history page adding a "Changes"
column.

No documentation was added, after the following discussion:
#1128 (comment)
Also prefixed the method with `_`, to mark it as not being part of the
public API.

Author: ddabble

simple_history/manager.py

@@ -91,6 +91,18 @@ def latest_of_each(self):
             )
         return latest_historics
 
+    def _select_related_history_tracked_objs(self):
+        """
+        A convenience method that calls ``select_related()`` with all the names of
+        the model's history-tracked ``ForeignKey`` fields.
+        """
+        field_names = [
+            field.name
+            for field in self.model.tracked_fields
+            if isinstance(field, models.ForeignKey)
+        ]
+        return self.select_related(*field_names)
+
     def _clone(self):
         c = super()._clone()
         c._as_instances = self._as_instances

simple_history/tests/tests/test_manager.py

@@ -8,7 +8,7 @@
 
 from simple_history.manager import SIMPLE_HISTORY_REVERSE_ATTR_NAME
 
-from ..models import Document, Poll, RankedDocument
+from ..models import Choice, Document, Poll, RankedDocument
 
 User = get_user_model()
 
@@ -371,3 +371,37 @@ def test_bulk_history_create_with_change_reason(self):
                 ]
             )
         )
+
+
+class PrefetchingMethodsTestCase(TestCase):
+    def setUp(self):
+        d = datetime(3021, 1, 1, 10, 0)
+        self.poll1 = Poll.objects.create(question="why?", pub_date=d)
+        self.poll2 = Poll.objects.create(question="how?", pub_date=d)
+        self.choice1 = Choice.objects.create(poll=self.poll1, votes=1)
+        self.choice2 = Choice.objects.create(poll=self.poll1, votes=2)
+        self.choice3 = Choice.objects.create(poll=self.poll2, votes=3)
+
+    def test__select_related_history_tracked_objs__prefetches_expected_objects(self):
+        num_choices = Choice.objects.count()
+        self.assertEqual(num_choices, 3)
+
+        def access_related_objs(records):
+            for record in records:
+                self.assertIsInstance(record.poll, Poll)
+
+        # Without prefetching:
+        with self.assertNumQueries(1):
+            historical_records = Choice.history.all()
+            self.assertEqual(len(historical_records), num_choices)
+        with self.assertNumQueries(num_choices):
+            access_related_objs(historical_records)
+
+        # With prefetching:
+        with self.assertNumQueries(1):
+            historical_records = (
+                Choice.history.all()._select_related_history_tracked_objs()
+            )
+            self.assertEqual(len(historical_records), num_choices)
+        with self.assertNumQueries(0):
+            access_related_objs(historical_records)

simple_history/tests/tests/test_models.py

@@ -1151,6 +1151,14 @@ def assert_tracked_fields_equal(model, expected_field_names):
             PollWithHistoricalIPAddress,
             ["id", "question", "pub_date"],
         )
+        assert_tracked_fields_equal(
+            PollWithManyToMany,
+            ["id", "question", "pub_date"],
+        )
+        assert_tracked_fields_equal(
+            Choice,
+            ["id", "poll", "choice", "votes"],
+        )
         assert_tracked_fields_equal(
             ModelWithCustomAttrOneToOneField,
             ["id", "poll"],