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, 11 insertions, 2 deletions

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index 16df9bfd91..33a9e9e166 100644
--- a/django/db/migrations/autodetector.py
+++ b/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)