fix(mongo): ensure assign on object properties won't ignore changes

Closes #5158

packages/core/src/entity/EntityAssigner.ts

@@ -109,8 +109,7 @@ export class EntityAssigner {
     }
 
     if (options.mergeObjects && Utils.isPlainObject(entity[propName]) && Utils.isPlainObject(value)) {
-      entity[propName] ??= {};
-      Utils.merge(entity[propName], value);
+      entity[propName] = Utils.merge({}, entity[propName], value);
     } else if (!prop || prop.setter || !prop.getter) {
       entity[propName] = value;
     }

tests/features/entity-assigner/GH5158.test.ts

@@ -0,0 +1,66 @@
+import {
+  Entity,
+  JsonType,
+  PrimaryKey,
+  Property,
+} from '@mikro-orm/core';
+import { MikroORM, ObjectId } from '@mikro-orm/mongodb';
+
+@Entity()
+class User {
+
+  @PrimaryKey()
+  _id!: ObjectId;
+
+  @Property({ type: JsonType })
+  methods: { method1?: string; method2?: string } = {};
+
+}
+
+let orm: MikroORM;
+
+beforeAll(async () => {
+  orm = await MikroORM.init({
+    dbName: '5158',
+    entities: [User],
+  });
+});
+
+afterAll(async () => {
+  await orm.close();
+});
+
+test('should update json properties with assign()', async () => {
+  // initialize user
+  const user1 = new User();
+  user1.methods = { method1: '1', method2: '1' };
+  const em1 = orm.em.fork();
+  await em1.persistAndFlush(user1);
+
+  // verify that it was persisted properly
+  const emRead1 = orm.em.fork();
+  const result1 = await emRead1.findOneOrFail(User, user1._id);
+  expect(result1.methods).toEqual({ method1: '1', method2: '1' });
+
+  // updating JSON property without assign() should work
+  const em2 = orm.em.fork();
+  const user2 = await em2.findOneOrFail(User, user1._id);
+  user2.methods = { method1: '1', method2: '2' };
+  await em2.persistAndFlush(user2);
+
+  // verify that it was persisted properly
+  const emRead2 = orm.em.fork();
+  const result2 = await emRead2.findOneOrFail(User, user1._id);
+  expect(result2.methods).toEqual({ method1: '1', method2: '2' });
+
+  // updating JSON property with assign() should work
+  const em3 = orm.em.fork();
+  const user3 = await em3.findOneOrFail(User, user1._id);
+  em3.assign(user3, { methods: { method2: '3' } });
+  await em3.persistAndFlush(user3);
+
+  // verify that it was persisted properly
+  const emRead3 = orm.em.fork();
+  const result3 = await emRead3.findOneOrFail(User, user1._id);
+  expect(result3.methods).toEqual({ method1: '1', method2: '3' });
+});