fix(core): fix assignability of Loaded type to naked entity

Closes #3865

packages/core/src/typings.ts

@@ -693,7 +693,11 @@ type Defined<T> = Exclude<T, null | undefined>;
 //   1. It yes, mark the collection or reference loaded and resolve its inner type recursively (passing suffix).
 //   2. If no, just return it as-is (scalars will be included, loadables too but not loaded).
 export type Loaded<T, L extends string = never> = T & {
-  [K in keyof T as IsPrefixed<K, L>]: LoadedLoadable<T[K], Loaded<ExtractType<T[K]>, Suffix<L>>>;
+  // this feels more correct, but breaks serialization methods on base entity, see #3865
+  // [K in keyof T as IsPrefixed<K, L>]: LoadedLoadable<T[K], Loaded<ExtractType<T[K]>, Suffix<L>>>;
+  [K in keyof T]: K extends Prefix<L>
+    ? LoadedLoadable<T[K], Loaded<ExtractType<T[K]>, Suffix<L>>>
+    : T[K]
 };
 
 export interface LoadedReference<T> extends Reference<Defined<T>> {

tests/types.test.ts

@@ -1,5 +1,5 @@
-import { wrap } from '@mikro-orm/core';
-import type { IdentifiedReference, Reference, Collection, EntityManager, EntityName, RequiredEntityData } from '@mikro-orm/core';
+import { Ref, wrap } from '@mikro-orm/core';
+import type { BaseEntity, IdentifiedReference, Reference, Collection, EntityManager, EntityName, RequiredEntityData } from '@mikro-orm/core';
 import type { Has, IsExact } from 'conditional-type-checks';
 import { assert } from 'conditional-type-checks';
 import type { ObjectId } from 'bson';
@@ -517,4 +517,17 @@ describe('check typings', () => {
     assert<IsExact<typeof id22, string | undefined>>(true);
   });
 
+  test('Loaded type and assignability with extending the ORM BaseEntity (#3865)', async () => {
+    interface MemberNotification extends BaseEntity<MemberNotification, 'id'> {
+      id: string;
+      notification?: Ref<Notification>;
+    }
+
+    interface Notification extends BaseEntity<Notification, 'id'> {
+      id: string;
+    }
+
+    const test: MemberNotification = {} as Loaded<MemberNotification, 'notification'>;
+  });
+
 });