Fixed #29193 -- Prevented unnecessary foreign key drops when altering a unique field.

Stopped dropping and recreating foreign key constraints on other fields
in the same table as the one which is actually being altered in an
AlterField operation.

Regression in c3e0adcad8.

tests/schema/tests.py

@@ -1545,6 +1545,40 @@ class SchemaTests(TransactionTestCase):
             TagUniqueRename.objects.create(title="bar", slug2="foo")
         Tag.objects.all().delete()
 
+    @isolate_apps('schema')
+    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite naively remakes the table on field alteration.')
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_unique_no_unnecessary_fk_drops(self):
+        """
+        If AlterField isn't selective about dropping foreign key constraints
+        when modifying a field with a unique constraint, the AlterField
+        incorrectly drops and recreates the Book.author foreign key even though
+        it doesn't restrict the field being changed (#29193).
+        """
+        class Author(Model):
+            name = CharField(max_length=254, unique=True)
+
+            class Meta:
+                app_label = 'schema'
+
+        class Book(Model):
+            author = ForeignKey(Author, CASCADE)
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+        new_field = CharField(max_length=255, unique=True)
+        new_field.model = Author
+        new_field.set_attributes_from_name('name')
+        with patch_logger('django.db.backends.schema', 'debug') as logger_calls:
+            with connection.schema_editor() as editor:
+                editor.alter_field(Author, Author._meta.get_field('name'), new_field)
+            # One SQL statement is executed to alter the field.
+            self.assertEqual(len(logger_calls), 1)
+
     @isolate_apps('schema')
     @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite remakes the table on field alteration.')
     def test_unique_and_reverse_m2m(self):