1 files changed, 34 insertions, 0 deletions
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index cd7e4f638c..6046e4473d 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -1539,6 +1539,40 @@ class SchemaTests(TransactionTestCase):
         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):
         """