Ability to query interfaces that a union implements

[sdnts]

We're using genql's typings on an upcoming app and its been working really well! It fits right in, and typed queries and mutations are awesome!

There's one area where type support is limited though, and that has to do with interfaces and unions.
When querying against a resource that returns a union, you should also be able to query interfaces that the union's types implement. Currently, it is only possible to query individual types within the union.

Perhaps an example will make this clearer.

So assume this (part of) GraphQL schema

interface ClientError {
  message: String!
}

type ClientErrorNameAlreadyTaken implements ClientError {
  message: String!
}

type ClientErrorNameInvalid implements ClientError {
  message: String!
}

union GenericError =
    ClientErrorNameAlreadyTaken
  | ClientErrorNameInvalid

type Query {
  error: GenericError
}

Here, this query is currently possible:

query q {
  error {
    ... on ClientErrorNameAlreadyTaken { message }
    ... on ClientErrorNameInvalid { message }
  }
}

as

client.query({
  error: {
    on_ClientErrorNameAlreadyTaken: { message: true }
    on_ClientErrorNameInvalid: { message: true }
  }
})

However, the following is also a valid query in GraphQL that genql disallows:

query q {
  error {
    ... on ClientError { message }
  }
}

It'd be awesome if genql could let me query it this way:

client.query({
  error: {
    on_ClientError: { message: true }
  }
})

If I'm being unclear, please let me know!

I'm more than happy to send a PR your way to support this if you like :)

[remorses]

This is not that simple to implement, why don't you use the interface directly instead of the union?

interface ClientError {
  message: String!
}

type ClientErrorNameAlreadyTaken implements ClientError {
  message: String!
}

type ClientErrorNameInvalid implements ClientError {
  message: String!
}

type Query {
  error: ClientError
}

[sdnts]

So in our case, we'd like to eventually have some error-specific fields as well (the example was rather simplified). So let's say ClientErrorNameAlreadyTaken could return a suggested name as well.

The goal of my second query then, is to fetch common fields from a all types within the union.

I'll also let @jasonkuhrt chime in if he wants

[jasonkuhrt]

@remorses Because not every operation will return every kind of client error, but we want all client errors to share a base interface so that clients can do a catch-all on the errors of an operation because:

1. Convenience when going deeper on specific errors is not neeeded

2. Correctness when the API adds more client error cases to an operation. E.g. evolving the API. In this case the client naturally could not have previously selected for the newly added kind of client error. Hence the client should be using the catch all via the interface to guard against this possibility.

More details on this topic here: https://productionreadygraphql.com/2020-08-01-guide-to-graphql-errors

[jasonkuhrt]

This is not that simple to implement

Would you be open for a PR from us?

[remorses]

I will work on this, I already know what to change, i also need to clean up tests because now it's a mess

[remorses]

Released in 2.4.0

[sdnts]

@remorses Just tried this out, works exactly as expected! Thanks so much for getting this out so incredibly quickly!