[docs] NestJS serialization vs MikroORM serialization

[marcpicaud]

tldr;

As a MikroORM newbie using NestJS, I want to find in the docs a clear warning about the class-transformer incompatibility, so that I don't get stuck trying to follow NestJS serialization techniques.

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First of all, thanks for this amazing project.

I think we could improve the documentation regarding one aspect : serialization.

NestJS documentation about serialization is pushing developers towards class-transformer and interceptors.

In the context of MikroORM, this is misleading. Concepts like Collection, Reference etc are not compatible with class-transformer, and people who are following NestJS serialization techniques by the book are getting stuck (well, I was).

I think that this incompatibility is not an issue at all, the built-in serialization in MikroORM is equivalent. But the NestJS integration should display a clear warning about that.

Related links :

• Question: How to transform and validate MikroORM collections using class-transformer and class-validator? mikro-orm#720
• Entity relationships now break with class-transformer mikro-orm#2687
• feat: added entities serialization interceptor #38

What do you guys think ? Happy to submit a PR.

[B4nan]

PR welcome 👍

Note that serialization with class-transformer is possible, you just need to return the DTO, not entity (so wrap(e).toJSON()). Creating new entities from DTO is not possible, that needs to happen via em.create() or similar, but again, you can use class-transformer before you call em.create() to do any kind of validation on the DTO.

Maybe we could have some kind of extension for class-transformer?

[marcpicaud]

Note that serialization with class-transformer is possible, you just need to return the DTO, not entity (so wrap(e).toJSON()).

wrap(e).toJSON() returns a generic object, so Nest's ClassSerializerInterceptor will do nothing with it before sending back the HTTP response. This interceptor expects a class instance (with decorated properties like @Exclude(), @Expose() etc.).

The problem is not really with class-transformer, but more with ClassSerializerInterceptor (which is widely used).

[marcpicaud]

Ok I'll make a PR 👍

DTO ---> Entity :

For me there's no issue here in the NestJS context. It provides a ValidationPipe so that incoming DTOs are actually validated before they reach the API route handlers. Once there, creating the entity through em.create() and the incoming DTO is pretty straightforward.

Entity ---> DTO :

Here's my understanding of the possible solutions :

• As you suggested, developers can return the DTO instead of the wrapped entity. Easy to forget though. EDIT: I could not make this work
• We can also drop the ClassSerializerInterceptor and just use MikroORM serialization options in the @Property decorator. I prefer this way, there are less moving pieces and it does the job just fine (hide, expose, and transform properties)
• Find a way to automatically call .toJSON() on wrapped entities before the ClassSerializerInterceptor steps in. Maybe another NestJS interceptor upstream?

[B4nan]

Find a way to automatically call .toJSON() on wrapped entities before the ClassSerializerInterceptor steps in. Maybe another NestJS interceptor upstream?

That's probably what #38 is for.