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.

django/db/migrations/autodetector.py

@@ -848,8 +848,16 @@ class MigrationAutodetector(object):
             old_field_dec = self.deep_deconstruct(old_field)
             new_field_dec = self.deep_deconstruct(new_field)
             if old_field_dec != new_field_dec:
-                if (not isinstance(old_field, models.ManyToManyField) and
-                        not isinstance(new_field, models.ManyToManyField)):
+                both_m2m = (
+                    isinstance(old_field, models.ManyToManyField) and
+                    isinstance(new_field, models.ManyToManyField)
+                )
+                neither_m2m = (
+                    not isinstance(old_field, models.ManyToManyField) and
+                    not isinstance(new_field, models.ManyToManyField)
+                )
+                if both_m2m or neither_m2m:
+                    # Either both fields are m2m or neither is
                     preserve_default = True
                     if (old_field.null and not new_field.null and not new_field.has_default() and
                             not isinstance(new_field, models.ManyToManyField)):
@@ -870,6 +878,7 @@ class MigrationAutodetector(object):
                         )
                     )
                 else:
+                    # We cannot alter between m2m and concrete fields
                     self._generate_removed_field(app_label, model_name, field_name)
                     self._generate_added_field(app_label, model_name, field_name)
 

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