Implement oneOf for input types

[patrick91]

Might be worth implementing this upcoming feature, unions are not allowed in input types and this is a good alternative to that, also should be easy to implement in code 😊

https://stepzen.com/blog/coming-soon-to-graphql-the-oneof-input-object

It's also been implemented in other libraries, for example async GraphQL (a rust library)

From: #2420 (reply in thread)

The specs: https://github.com/graphql/graphql-spec/pull/825/files

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[nrbnlulu]

Would that be

@strawberry.mutations.oneOf()
def foo(arg1: Optional[str], arg2: Optional[int]):
    ...

or use enums like async-graphql?

[Aponace]

Any progress on that? It seems the PR is open and ready but no one approves it :(

[patrick91]

@Aponace while the PR adds the directive, it doesn't add runtime support for it, we'd need to implement that to make it work 😊

[bradleyoesch]

@patrick91 I'm interested in implementing this!

It seems like we could take #2560 to add @oneOf as a schema directive, and maybe we could add a ValidationRule that is added to all schemas? Since this directive would apply on execution, feels more appropriate for a "native" extension or something similar to operation directives.

Seems like the logic is straightforward, just not 100% sure on where the implementation should live.