Populating Nested Field returns the wrong number of associated entities

[thiagomini]

Describe the bug

• Given we are using Entity Schema definitions
• And we have a Company entity class with its schema
• And a Company has many User's
• And a User has many Profile's
• And we have a Company "Acme" that has "User1" and "User2" as users
• And "User1" has two profiles
• And "User2" has one profile
• When we try to fetch the Company from the database, populating both user and profile
• Then the "User2" has three profiles associated.

The Diagram below illustrates that:

flowchart LR
    A[Company 'A'] -->|Has| B(User 1)
    A[Company 'A'] -->|Has| C(User 2)
    B --> |Has| D(Profile 1)  
    B --> |Has| E(Profile 2)  
    C --> |Has| F(Profile 3)

Stack trace

AssertionError: expected { Object (0, 1, ...) } to have a length of 1 but got 3

- Expected
+ Received

- 1
+ 3

 ❯ src/mikro-orm/mikro-orm-internal.module.spec.ts:100:45
     98|     // Assert
     99|     const userWithASingleProfile = companyWithProfiles.users.find(u => u.id === user2.id);
    100|     expect(userWithASingleProfile.profiles).toHaveLength(1);
       |                                             ^
    101|   });
    102| 

To Reproduce Steps to reproduce the behavior:

1. Clone the minimum reproducible code repository: https://github.com/thiagomini/nest-mikro-orm-example/tree/bug/nested-populated-entities
2. run yarn
3. run docker compose up -d
4. run yarn test

Expected behavior "User2" should have only ONE associated Profile.

Additional Information

💡 Link to the test case

Versions

Dependency	Version
node	18.12.1
typescript	5.2.2
mikro-orm	5.8.3
pg	8.11.2

[thiagomini]

Hey, @B4nan , this is either some lousy mistake on my part or an important issue to fix - It looks like populating nested fields is making the wrong associations, which might cause severe bugs 🤔 . I appreciate it if you could help me out with this one!

💡 Edit: I noticed that if we manually initialize each user's profile collection, it returns the correct data. The issue is probably related to the EntityManager - I'd guess it's not filtering which grand-children entities belong to each child entity - As you can see, it's basically grouping all of the existing profiles (3 in total) and associating them with each user.

[thiagomini]

@B4nan Hey again Martin, sorry for bothering you again with this, but that issue has been blocking a bit of our progress in a project here. Could you give us some insight? Maybe just pointing out where in the MikroORM code we could try to fix that issue. Thank you!

[B4nan]

Can you create repro without nest js? Just a simple script, ideally everything in a single file.

[thiagomini]

Alright, I'll work on it, thanks Martin!

[thiagomini]

Hey again, @B4nan! Here's the Repro: https://github.com/thiagomini/mikro-orm-repro

And here's the test case: https://github.com/thiagomini/mikro-orm-repro/blob/f48a1d83f8ea9d13da0f0e6f54645a3c7533f2d4/src/mikro-orm.test.ts#L32C2-L32C2

So, apparently, the issue might indeed be with the NestJS package because the very same test here is actually passing 😅 - I'll create another version tomorrow using vitest instead of jest, just to rule out that possibility.

⚠️ Edit: I've added a yarn vitest command that executes it instead of jest, and the test also passes. To it is definitely the Nest package!