Fixed #30591 -- Fixed recreation of foreign key constraints on MySQL when altering type of referenced unique field.

Thanks Mariusz Felisiak for tests and Matthijs Kooijman for
investigation and initial patch.

1 files changed, 7 insertions, 3 deletions

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 3a0dc39238..98afbcc05a 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -28,12 +28,16 @@ def _is_relevant_relation(relation, altered_field):
     return altered_field.name in field.to_fields
 
 
+def _all_related_fields(model):
+    return model._meta._get_fields(forward=False, reverse=True, include_hidden=True)
+
+
 def _related_non_m2m_objects(old_field, new_field):
     # Filter out m2m objects from reverse relations.
     # Return (old_relation, new_relation) tuples.
     return zip(
-        (obj for obj in old_field.model._meta.related_objects if _is_relevant_relation(obj, old_field)),
-        (obj for obj in new_field.model._meta.related_objects if _is_relevant_relation(obj, new_field))
+        (obj for obj in _all_related_fields(old_field.model) if _is_relevant_relation(obj, old_field)),
+        (obj for obj in _all_related_fields(new_field.model) if _is_relevant_relation(obj, new_field)),
     )
 
 
@@ -753,7 +757,7 @@ class BaseDatabaseSchemaEditor:
         # Type alteration on primary key? Then we need to alter the column
         # referring to us.
         rels_to_update = []
-        if old_field.primary_key and new_field.primary_key and old_type != new_type:
+        if drop_foreign_keys:
             rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
         # Changed to become primary key?
         if self._field_became_primary_key(old_field, new_field):