Tested altering a unique field when a reverse M2M relation exists.

tests/schema/tests.py

@@ -22,7 +22,7 @@ from django.db.transaction import TransactionManagementError, atomic
 from django.test import (
     TransactionTestCase, skipIfDBFeature, skipUnlessDBFeature,
 )
-from django.test.utils import CaptureQueriesContext, isolate_apps
+from django.test.utils import CaptureQueriesContext, isolate_apps, patch_logger
 from django.utils import timezone
 
 from .fields import (
@@ -1545,6 +1545,42 @@ class SchemaTests(TransactionTestCase):
             TagUniqueRename.objects.create(title="bar", slug2="foo")
         Tag.objects.all().delete()
 
+    @isolate_apps('schema')
+    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite remakes the table on field alteration.')
+    def test_unique_and_reverse_m2m(self):
+        """
+        AlterField can modify a unique field when there's a reverse M2M
+        relation on the model.
+        """
+        class Tag(Model):
+            title = CharField(max_length=255)
+            slug = SlugField(unique=True)
+
+            class Meta:
+                app_label = 'schema'
+
+        class Book(Model):
+            tags = ManyToManyField(Tag, related_name='books')
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Tag)
+            editor.create_model(Book)
+        new_field = SlugField(max_length=75, unique=True)
+        new_field.model = Tag
+        new_field.set_attributes_from_name('slug')
+        with patch_logger('django.db.backends.schema', 'debug') as logger_calls:
+            with connection.schema_editor() as editor:
+                editor.alter_field(Tag, Tag._meta.get_field('slug'), new_field)
+            # One SQL statement is executed to alter the field.
+            self.assertEqual(len(logger_calls), 1)
+        # Ensure that the field is still unique.
+        Tag.objects.create(title='foo', slug='foo')
+        with self.assertRaises(IntegrityError):
+            Tag.objects.create(title='bar', slug='foo')
+
     def test_unique_together(self):
         """
         Tests removing and adding unique_together constraints on a model.