[1.7.x] Fixed #22476: Couldn't alter attributes on M2Ms with through= set

1 files changed, 4 insertions, 1 deletions

diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index 005d4b13c8..5b6155be47 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -148,8 +148,11 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         new_type = new_db_params['type']
         if old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and old_field.rel.through._meta.auto_created and new_field.rel.through._meta.auto_created):
             return self._alter_many_to_many(model, old_field, new_field, strict)
+        elif old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and not old_field.rel.through._meta.auto_created and not new_field.rel.through._meta.auto_created):
+            # Both sides have through models; this is a no-op.
+            return
         elif old_type is None or new_type is None:
-            raise ValueError("Cannot alter field %s into %s - they are not compatible types (probably means only one is an M2M with implicit through model)" % (
+            raise ValueError("Cannot alter field %s into %s - they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)" % (
                 old_field,
                 new_field,
             ))