Fixed #23738 -- Allowed migrating from NULL to NOT NULL with the same default value

Thanks to Andrey Antukh for the report.

django/db/backends/schema.py

@@ -607,7 +607,7 @@ class BaseDatabaseSchemaEditor(object):
                 # directly run a (NOT) NULL alteration
                 actions = actions + null_actions
             # Combine actions together if we can (e.g. postgres)
-            if self.connection.features.supports_combined_alters:
+            if self.connection.features.supports_combined_alters and actions:
                 sql, params = tuple(zip(*actions))
                 actions = [(", ".join(sql), reduce(operator.add, params))]
             # Apply those actions

docs/releases/1.7.2.txt

@@ -37,3 +37,6 @@ Bugfixes
 
 * Added support for transactional spatial metadata initialization on
   SpatiaLite 4.1+ (:ticket:`23152`).
+
+* Fixed a migration crash that prevented changing a nullable field with a
+  default to non-nullable with the same default (:ticket:`23738`).

tests/schema/models.py

@@ -17,6 +17,14 @@ class Author(models.Model):
         apps = new_apps
 
 
+class AuthorWithDefaultHeight(models.Model):
+    name = models.CharField(max_length=255)
+    height = models.PositiveIntegerField(null=True, blank=True, default=42)
+
+    class Meta:
+        apps = new_apps
+
+
 class AuthorWithM2M(models.Model):
     name = models.CharField(max_length=255)
 

tests/schema/tests.py

@@ -7,7 +7,7 @@ from django.db.models.fields import (BinaryField, BooleanField, CharField, Integ
     PositiveIntegerField, SlugField, TextField)
 from django.db.models.fields.related import ManyToManyField, ForeignKey
 from django.db.transaction import atomic
-from .models import (Author, AuthorWithM2M, Book, BookWithLongName,
+from .models import (Author, AuthorWithDefaultHeight, AuthorWithM2M, Book, BookWithLongName,
     BookWithSlug, BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
     UniqueTest, Thing, TagThrough, BookWithM2MThrough, AuthorTag, AuthorWithM2MThrough,
     AuthorWithEvenLongerName, BookWeak)
@@ -448,6 +448,31 @@ class SchemaTests(TransactionTestCase):
         self.assertEqual(Author.objects.get(name='Not null author').height, 12)
         self.assertEqual(Author.objects.get(name='Null author').height, 42)
 
+    @unittest.skipUnless(connection.features.supports_combined_alters, "No combined ALTER support")
+    def test_alter_null_to_not_null_keeping_default(self):
+        """
+        #23738 - Can change a nullable field with default to non-nullable
+        with the same default.
+        """
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithDefaultHeight)
+        # Ensure the field is right to begin with
+        columns = self.column_classes(AuthorWithDefaultHeight)
+        self.assertTrue(columns['height'][1][6])
+        # Alter the height field to NOT NULL keeping the previous default
+        new_field = PositiveIntegerField(default=42)
+        new_field.set_attributes_from_name("height")
+        with connection.schema_editor() as editor:
+            editor.alter_field(
+                AuthorWithDefaultHeight,
+                AuthorWithDefaultHeight._meta.get_field_by_name("height")[0],
+                new_field,
+            )
+        # Ensure the field is right afterwards
+        columns = self.column_classes(AuthorWithDefaultHeight)
+        self.assertFalse(columns['height'][1][6])
+
     @unittest.skipUnless(connection.features.supports_foreign_keys, "No FK support")
     def test_alter_fk(self):
         """