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.

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 _all_related_fields(old_field.model) 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(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):

tests/migrations/test_operations.py

@@ -1308,6 +1308,51 @@ class OperationTests(OperationTestBase):
             operation.database_backwards("test_alflpkfk", editor, new_state, project_state)
         assertIdTypeEqualsFkType()
 
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_alter_field_reloads_state_on_fk_with_to_field_target_type_change(self):
+        app_label = 'alter_field_reloads_state_on_fk_with_to_field_target_type_change'
+        project_state = self.apply_operations(app_label, ProjectState(), operations=[
+            migrations.CreateModel('Rider', fields=[
+                ('id', models.AutoField(primary_key=True)),
+                ('code', models.PositiveIntegerField(unique=True)),
+            ]),
+            migrations.CreateModel('Pony', fields=[
+                ('id', models.AutoField(primary_key=True)),
+                ('rider', models.ForeignKey('%s.Rider' % app_label, models.CASCADE, to_field='code')),
+            ]),
+        ])
+        operation = migrations.AlterField(
+            'Rider',
+            'code',
+            models.CharField(max_length=100, unique=True),
+        )
+        self.apply_operations(app_label, project_state, operations=[operation])
+
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_alter_field_reloads_state_on_fk_with_to_field_related_name_target_type_change(self):
+        app_label = 'alter_field_reloads_state_on_fk_with_to_field_rn_target_type_change'
+        project_state = self.apply_operations(app_label, ProjectState(), operations=[
+            migrations.CreateModel('Rider', fields=[
+                ('id', models.AutoField(primary_key=True)),
+                ('code', models.PositiveIntegerField(unique=True)),
+            ]),
+            migrations.CreateModel('Pony', fields=[
+                ('id', models.AutoField(primary_key=True)),
+                ('rider', models.ForeignKey(
+                    '%s.Rider' % app_label,
+                    models.CASCADE,
+                    to_field='code',
+                    related_name='+',
+                )),
+            ]),
+        ])
+        operation = migrations.AlterField(
+            'Rider',
+            'code',
+            models.CharField(max_length=100, unique=True),
+        )
+        self.apply_operations(app_label, project_state, operations=[operation])
+
     def test_alter_field_reloads_state_on_fk_target_changes(self):
         """
         If AlterField doesn't reload state appropriately, the second AlterField