From 8f04473af1da1ed4e8a07e89205da4e70d69b586 Mon Sep 17 00:00:00 2001
From: David Wobrock <david.wobrock@gmail.com>
Date: Mon, 2 May 2022 14:43:35 +0200
Subject: [PATCH] Fixed #25105 -- Checked deferred constraints before updating
 rows on PostgreSQL.

---
 django/db/backends/postgresql/schema.py |  7 ++++++
 tests/schema/tests.py                   | 32 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index 3053c8d370..a22ae0094f 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -7,6 +7,13 @@ from django.db.backends.utils import strip_quotes
 
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
+    # Setting all constraints to IMMEDIATE to allow changing data in the same
+    # transaction.
+    sql_update_with_default = (
+        "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
+        "; SET CONSTRAINTS ALL IMMEDIATE"
+    )
+
     sql_delete_sequence = "DROP SEQUENCE IF EXISTS %(sequence)s CASCADE"
 
     sql_create_index = (
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 2d12796cbd..fe717f8841 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -973,6 +973,38 @@ class SchemaTests(TransactionTestCase):
             Node.objects.update(parent=parent)
             editor.alter_field(Node, old_field, new_field, strict=True)
 
+    @isolate_apps("schema")
+    def test_alter_null_with_default_value_deferred_constraints(self):
+        class Publisher(Model):
+            class Meta:
+                app_label = "schema"
+
+        class Article(Model):
+            publisher = ForeignKey(Publisher, CASCADE)
+            title = CharField(max_length=50, null=True)
+            description = CharField(max_length=100, null=True)
+
+            class Meta:
+                app_label = "schema"
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Publisher)
+            editor.create_model(Article)
+        self.isolated_local_models = [Article, Publisher]
+
+        publisher = Publisher.objects.create()
+        Article.objects.create(publisher=publisher)
+
+        old_title = Article._meta.get_field("title")
+        new_title = CharField(max_length=50, null=False, default="")
+        new_title.set_attributes_from_name("title")
+        old_description = Article._meta.get_field("description")
+        new_description = CharField(max_length=100, null=False, default="")
+        new_description.set_attributes_from_name("description")
+        with connection.schema_editor() as editor:
+            editor.alter_field(Article, old_title, new_title, strict=True)
+            editor.alter_field(Article, old_description, new_description, strict=True)
+
     def test_alter_text_field_to_date_field(self):
         """
         #25002 - Test conversion of text field to date field.