fix(core): hydrate collection items when merging data in `factory.cre…

…ate`

Closes mikro-orm#4173

tests/issues/GH4173.test.ts

@@ -0,0 +1,72 @@
+import { Collection, Entity, ManyToOne, OneToMany, PrimaryKey } from '@mikro-orm/core';
+import { MikroORM } from '@mikro-orm/sqlite';
+
+@Entity()
+export class User {
+
+  @PrimaryKey()
+  id!: string;
+
+  @OneToMany(() => Post, post => post.user)
+  posts = new Collection<Post>(this);
+
+}
+
+@Entity()
+export class Post {
+
+  @PrimaryKey()
+  id!: string;
+
+  @OneToMany(() => Comment, comment => comment.post)
+  comments = new Collection<Comment>(this);
+
+  @ManyToOne()
+  user!: User;
+
+}
+
+@Entity()
+class Comment {
+
+  @PrimaryKey()
+  id!: string;
+
+  @ManyToOne()
+  post!: Post;
+
+}
+
+let orm: MikroORM;
+
+beforeAll(async () => {
+  orm = await MikroORM.init({
+    dbName: ':memory:',
+    entities: [Comment, Post, User],
+  });
+  await orm.schema.refreshDatabase();
+  orm.em.create(User, { id: 'user1' });
+  orm.em.create(Post, { id: 'post1', user: 'user1' });
+  orm.em.create(Comment, { id: 'comment', post: 'post1' });
+  await orm.em.flush();
+});
+
+beforeEach(() => orm.em.clear());
+afterAll(() => orm.close(true));
+
+test('it should select comments', async () => {
+  const users2 = await orm.em.qb(User)
+    .select('*')
+    .joinAndSelect('posts', 'p')
+    .leftJoinAndSelect('p.comments', 'c');
+  expect(users2[0].posts[0].comments[0]).toBeDefined();
+});
+
+test('it should select comments even if posts have already been selected', async () => {
+  await orm.em.qb(Post).select('*');
+  const users1 = await orm.em.qb(User)
+    .select('*')
+    .joinAndSelect('posts', 'p')
+    .leftJoinAndSelect('p.comments', 'c');
+  expect(users1[0].posts[0].comments[0]).toBeDefined();
+});