[1.8.x] Fixed #24435 -- Prevented m2m field removal and addition in migrations when changing blank

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

Backport of a9e29fae10 from master

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