Refs #28010 -- Allowed reverse related fields in SELECT FOR UPDATE .. OF.

Thanks Adam Chidlow for polishing the patch.

django/db/models/sql/compiler.py

@@ -783,6 +783,7 @@ class SQLCompiler:
             klass_info = {
                 'model': f.remote_field.model,
                 'field': f,
+                'reverse': False,
                 'local_setter': f.set_cached_value,
                 'remote_setter': f.remote_field.set_cached_value if f.unique else lambda x, y: None,
                 'from_parent': False,
@@ -821,6 +822,7 @@ class SQLCompiler:
                 klass_info = {
                     'model': model,
                     'field': f,
+                    'reverse': True,
                     'local_setter': f.remote_field.set_cached_value,
                     'remote_setter': f.set_cached_value,
                     'from_parent': from_parent,
@@ -858,6 +860,7 @@ class SQLCompiler:
                     klass_info = {
                         'model': model,
                         'field': f,
+                        'reverse': True,
                         'local_setter': local_setter,
                         'remote_setter': remote_setter,
                         'from_parent': from_parent,
@@ -905,7 +908,10 @@ class SQLCompiler:
                     path = []
                     yield 'self'
                 else:
-                    path = parent_path + [klass_info['field'].name]
+                    field = klass_info['field']
+                    if klass_info['reverse']:
+                        field = field.remote_field
+                    path = parent_path + [field.name]
                     yield LOOKUP_SEP.join(path)
                 queue.extend(
                     (path, klass_info)
@@ -918,7 +924,10 @@ class SQLCompiler:
             klass_info = self.klass_info
             for part in parts:
                 for related_klass_info in klass_info.get('related_klass_infos', []):
-                    if related_klass_info['field'].name == part:
+                    field = related_klass_info['field']
+                    if related_klass_info['reverse']:
+                        field = field.remote_field
+                    if field.name == part:
                         klass_info = related_klass_info
                         break
                 else:

docs/ref/models/querysets.txt

@@ -1628,6 +1628,19 @@ specify the related objects you want to lock in ``select_for_update(of=(...))``
 using the same fields syntax as :meth:`select_related`. Use the value ``'self'``
 to refer to the queryset's model.
 
+You can't use ``select_for_update()`` on nullable relations::
+
+    >>> Person.objects.select_related('hometown').select_for_update()
+    Traceback (most recent call last):
+    ...
+    django.db.utils.NotSupportedError: FOR UPDATE cannot be applied to the nullable side of an outer join
+
+To avoid that restriction, you can exclude null objects if you don't care about
+them::
+
+    >>> Person.objects.select_related('hometown').select_for_update().exclude(hometown=None)
+    <QuerySet [<Person: ...)>, ...]>
+
 Currently, the ``postgresql``, ``oracle``, and ``mysql`` database
 backends support ``select_for_update()``. However, MySQL doesn't support the
 ``nowait``, ``skip_locked``, and ``of`` arguments.

tests/filtered_relation/tests.py

@@ -1,4 +1,4 @@
-from django.db import connection
+from django.db import connection, transaction
 from django.db.models import Case, Count, F, FilteredRelation, Q, When
 from django.test import TestCase
 from django.test.testcases import skipUnlessDBFeature
@@ -62,6 +62,20 @@ class FilteredRelationTests(TestCase):
                 (self.book4, self.author1),
             ], lambda x: (x, x.author_join))
 
+    @skipUnlessDBFeature('has_select_for_update', 'has_select_for_update_of')
+    def test_select_related_foreign_key_for_update_of(self):
+        with transaction.atomic():
+            qs = Book.objects.annotate(
+                author_join=FilteredRelation('author'),
+            ).select_related('author_join').select_for_update(of=('self',)).order_by('pk')
+            with self.assertNumQueries(1):
+                self.assertQuerysetEqual(qs, [
+                    (self.book1, self.author1),
+                    (self.book2, self.author2),
+                    (self.book3, self.author2),
+                    (self.book4, self.author1),
+                ], lambda x: (x, x.author_join))
+
     def test_without_join(self):
         self.assertSequenceEqual(
             Author.objects.annotate(

tests/select_for_update/models.py

@@ -14,3 +14,7 @@ class Person(models.Model):
     name = models.CharField(max_length=30)
     born = models.ForeignKey(City, models.CASCADE, related_name='+')
     died = models.ForeignKey(City, models.CASCADE, related_name='+')
+
+
+class PersonProfile(models.Model):
+    person = models.OneToOneField(Person, models.CASCADE, related_name='profile')

tests/select_for_update/tests.py

@@ -15,7 +15,7 @@ from django.test import (
 )
 from django.test.utils import CaptureQueriesContext
 
-from .models import City, Country, Person
+from .models import City, Country, Person, PersonProfile
 
 
 class SelectForUpdateTests(TransactionTestCase):
@@ -30,6 +30,7 @@ class SelectForUpdateTests(TransactionTestCase):
         self.city1 = City.objects.create(name='Liberchies', country=self.country1)
         self.city2 = City.objects.create(name='Samois-sur-Seine', country=self.country2)
         self.person = Person.objects.create(name='Reinhardt', born=self.city1, died=self.city2)
+        self.person_profile = PersonProfile.objects.create(person=self.person)
 
         # We need another database connection in transaction to test that one
         # connection issuing a SELECT ... FOR UPDATE will block.
@@ -225,13 +226,27 @@ class SelectForUpdateTests(TransactionTestCase):
         msg = (
             'Invalid field name(s) given in select_for_update(of=(...)): %s. '
             'Only relational fields followed in the query are allowed. '
-            'Choices are: self, born.'
+            'Choices are: self, born, profile.'
         )
         for name in ['born__country', 'died', 'died__country']:
             with self.subTest(name=name):
                 with self.assertRaisesMessage(FieldError, msg % name):
                     with transaction.atomic():
-                        Person.objects.select_related('born').select_for_update(of=(name,)).get()
+                        Person.objects.select_related(
+                            'born', 'profile',
+                        ).exclude(profile=None).select_for_update(of=(name,)).get()
+
+    @skipUnlessDBFeature('has_select_for_update', 'has_select_for_update_of')
+    def test_reverse_one_to_one_of_arguments(self):
+        """
+        Reverse OneToOneFields may be included in of=(...) as long as NULLs
+        are excluded because LEFT JOIN isn't allowed in SELECT FOR UPDATE.
+        """
+        with transaction.atomic():
+            person = Person.objects.select_related(
+                'profile',
+            ).exclude(profile=None).select_for_update(of=('profile',)).get()
+            self.assertEqual(person.profile, self.person_profile)
 
     @skipUnlessDBFeature('has_select_for_update')
     def test_for_update_after_from(self):