Entity relations are not serialized properly when calling toJSON on flushed entities after being created with EntityRepository.create

[charmpitz]

Describe the bug
When using toJSON on an Entity after it is being created, persisted and flushed via the create and persistAndFlush methods on the specific repository, its related entities are not being serialized properly. Instead of getting the result of toJSON being called on them we get a number representing their id.

Stack trace

function create(input) {
    ...

    let articleEntity: ArticleEntity = this.articleRepository.create({
        ...input,
        user: userEntity,
        tags,
    });

    await this.articleRepository.persistAndFlush(articleEntity);
    console.log(articleEntity.toJSON())
    /* Wrong Output:
    {
        id: 9,
        title: 'Some Article',
        tags: [ 1 ],
        user: 1
    }
    */

    articleEntity = await this.articleRepository.findOne(articleEntity.id)
    console.log(articleEntity.toJSON())
    /* Right Output:
    {
        id: 9,
        title: 'Some Article',
        tags: [
            {
                id: 1,
                name: 'tag1',
                user: [Object],
                ...
            }
        ],
        user: {
            id: 1
            username: 'someusername'
            ...
        }
    }
    */

    return articleEntity.toJSON() as Article;
}

To Reproduce
See above.

Expected behavior
I'm expecting for the articleEntity.toJSON() to return

    {
        id: 9,
        title: 'Some Article',
        tags: [
            {
                id: 1,
                name: 'tag1',
                user: [Object],
                ...
            }
        ],
        user: {
            id: 1,
            username: 'someusername',
            ...
        }
    }

without the need of calling findOne after flushing.

Additional context

Versions

Dependency	Version
node	14.7.0
@mikro-orm/cli	^4.0.0-rc.4
@mikro-orm/core	^4.0.0-rc.4
@mikro-orm/migrations	^4.0.0-rc.4
@mikro-orm/nestjs	^4.0.0-alpha.3
@mikro-orm/postgresql	^4.0.0-rc.4
@mikro-orm/reflection	^4.0.0-rc.4
typescript	^3.7.4

[B4nan]

This is actually quite huge breaking change, so will make this hidden behind a config toggle, keeping the default behaviour as is.

[B4nan]

In rc.6 you can set populateAfterFlush: true to enable this behaviour.

[charmpitz]

Sounds good, thank you for implementing it! ✌️

[SpencerKaiser]

@B4nan I just stumbled across this issue after commenting on #1861 (comment).

I have populateAfterFlush set to false (and it seems that's the default anyways) but my entity is still populating after flush... any idea what I'm doing wrong?

[B4nan]

I have populateAfterFlush set to false (and it seems that's the default anyways) but my entity is still populating after flush... any idea what I'm doing wrong?

No idea, provide a reproduction if you need help, as you might be facing some other problem (and yes, populateAfterFlush defaults to false)

[B4nan]

The default will change to true in v5