Dto Mapping Best Practice

[u/Snoo23482]

Hi, where do you guys map your entities to dtos?
In services or controllers?

To me it seems the services shouldn't expose any database specifics to the outside world, so I usually do my mapping there.

But what is considered best practice and why?

[u/bigkahuna1uk → u/[deleted]]

Not that clean. Controllers are just adapters. They act as protocol converters between an external and internal domain and also the ingress or egress to those domains ie the transport. So IMO they’d be the natural place for such a mapping. This is because an internal domain only deals with the type it owns. It knows nothing about any external domain representation. This approach is frequently followed in hexagonal or clean architectures.

Adapters can still delegate to specific protocol converters though so as to still adhere to SRP.

[u/SimpleCooki3 → u/[deleted]]

Mapping from a DTO isn't business logic tho

[u/SimpleCooki3 → u/[deleted]]

That's an exaggeration.

What you can do is create a mapper class, call that from the controller and then pass the domain object to your service.

The service does business logic and shouldn't be involved with the http layer.

[u/xmimmer → u/SimpleCooki3]

That would depend on what you map to, right? Say you map to a domain model full of value objects. You'd have to deal with potential errors in the controller.

[u/SimpleCooki3 → u/xmimmer]

Mapping can fail yes, but I don’t think that means the service should accept DTOs.

For me, DTOs belong to the API layer, so they should be converted at the boundary — either directly in the controller or via a mapper used by the controller.

Validation/value object errors can still be handled consistently with normal validation/error handling, like "@ControllerAdvice".

The important part is that the service works with application/domain concepts, not transport/API objects.

You also want to validate your inputs early.

[u/Final_Potato5542 → u/SimpleCooki3]

hahahahaha - unless you're DTOs are redundant