Fixed #24435 -- Prevented m2m field removal and addition in migrations when changing blank

Thanks Mark Tranchant for the report an Tim Graham for the test and
review.

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index 3d822072ce..2d45dac7f8 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -127,6 +127,10 @@ class AutodetectorTests(TestCase):
         ("id", models.AutoField(primary_key=True)),
         ("publishers", models.ManyToManyField("testapp.Publisher")),
     ])
+    author_with_m2m_blank = ModelState("testapp", "Author", [
+        ("id", models.AutoField(primary_key=True)),
+        ("publishers", models.ManyToManyField("testapp.Publisher", blank=True)),
+    ])
     author_with_m2m_through = ModelState("testapp", "Author", [
         ("id", models.AutoField(primary_key=True)),
         ("publishers", models.ManyToManyField("testapp.Publisher", through="testapp.Contract")),
@@ -1263,6 +1267,16 @@ class AutodetectorTests(TestCase):
         self.assertOperationTypes(changes, 'testapp', 0, ["AddField"])
         self.assertOperationAttributes(changes, 'testapp', 0, 0, name="publishers")
 
+    def test_alter_many_to_many(self):
+        before = self.make_project_state([self.author_with_m2m, self.publisher])
+        after = self.make_project_state([self.author_with_m2m_blank, self.publisher])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        # Right number/type of migrations?
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ["AlterField"])
+        self.assertOperationAttributes(changes, 'testapp', 0, 0, name="publishers")
+
     def test_create_with_through_model(self):
         """
         Adding a m2m with a through model and the models that use it should be