REST API code/message for missing parent resource

Question

I'm looking for some guidance for the correct response code & message when requesting for a resource that forms part of another resource.

For example, a GET request on:

users/{id}

where the user does not exist would return a 404, with a message of user resource not found.

My question is, what should the following return when no user resource is found:

users/{id}/friends

I'm currently returning the same code/message as in the first example. Should I be returning a message relating specifically to the friends resource? I personally think it's more helpful to make the API client aware that the parent resource isn't found, incase you have a larger URI chain.

Answer 1

I’d be tempted to return a 400 Bad Request header, and place the error message in the body of the response. Unfortunately there’s no right or wrong answer in this scenario, so go with what works best for you and your application.

Answer 2

In this particular example, if the point is to let the client differentiate between a friends request for a non-existent user, and a friends request for a user who simply has no friends, I think it would make the most sense to return 404 in the first case, and 200 with an empty set in the second.

In other words, "none" is a valid value for friends. There's no case where a user exists but their (potentially empty) list of friends doesn't, so there's never any ambiguity in issuing a 404 for the parent resource.