From 325d7710ce9f6155bb55610ad6b4580d31263557 Mon Sep 17 00:00:00 2001
From: David Wobrock <david.wobrock@gmail.com>
Date: Sat, 24 Jul 2021 08:17:39 +0200
Subject: [PATCH] Fixed #32743 -- Added foreign key altering when altering type
 of referenced primary key with MTI.

---
 django/db/backends/base/schema.py   |  8 ++++-
 tests/migrations/test_operations.py | 51 +++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 8050441414..47fdae8fd4 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -36,10 +36,16 @@ def _all_related_fields(model):
 def _related_non_m2m_objects(old_field, new_field):
     # Filter out m2m objects from reverse relations.
     # Return (old_relation, new_relation) tuples.
-    return zip(
+    related_fields = zip(
         (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)),
     )
+    for old_rel, new_rel in related_fields:
+        yield old_rel, new_rel
+        yield from _related_non_m2m_objects(
+            old_rel.remote_field,
+            new_rel.remote_field,
+        )
 
 
 class BaseDatabaseSchemaEditor:
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index af9f371da6..84f1b89ba5 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -1505,6 +1505,57 @@ class OperationTests(OperationTestBase):
             operation.database_backwards("test_alflpkfk", editor, new_state, project_state)
         assertIdTypeEqualsFkType()
 
+    def test_alter_field_pk_mti_fk(self):
+        app_label = 'test_alflpkmtifk'
+        project_state = self.set_up_test_model(app_label, mti_model=True)
+        project_state = self.apply_operations(app_label, project_state, [
+            migrations.CreateModel('ShetlandRider', fields=[
+                (
+                    'pony',
+                    models.ForeignKey(f'{app_label}.ShetlandPony', models.CASCADE),
+                ),
+            ]),
+        ])
+        operation = migrations.AlterField(
+            'Pony',
+            'id',
+            models.BigAutoField(primary_key=True),
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertIsInstance(
+            new_state.models[app_label, 'pony'].fields['id'],
+            models.BigAutoField,
+        )
+
+        def _get_column_id_type(cursor, table, column):
+            return [
+                c.type_code
+                for c in connection.introspection.get_table_description(
+                    cursor,
+                    f'{app_label}_{table}',
+                )
+                if c.name == column
+            ][0]
+
+        def assertIdTypeEqualsMTIFkType():
+            with connection.cursor() as cursor:
+                parent_id_type = _get_column_id_type(cursor, 'pony', 'id')
+                child_id_type = _get_column_id_type(cursor, 'shetlandpony', 'pony_ptr_id')
+                mti_id_type = _get_column_id_type(cursor, 'shetlandrider', 'pony_id')
+            self.assertEqual(parent_id_type, child_id_type)
+            self.assertEqual(parent_id_type, mti_id_type)
+
+        assertIdTypeEqualsMTIFkType()
+        # Alter primary key.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        assertIdTypeEqualsMTIFkType()
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        assertIdTypeEqualsMTIFkType()
+
     @skipUnlessDBFeature('supports_foreign_keys')
     def test_alter_field_reloads_state_on_fk_with_to_field_target_type_change(self):
         app_label = 'test_alflrsfkwtflttc'