GraphQL issues with __typename and interfaces/unions

[Patrick Stoica]

I'm trying to use this library to mock Apollo and am running into a few issues when using the generics (.query<Query, Variables>):

1. __typename is required and expects a predetermined value. String is a superset and won't work.
   • Workaround: __typename: () => "User" as const,
2. Interfaces (and possibly unions).. are a bit odd.
   • Some of these fields are only present in some implementations. .create has no awareness of this, but I do expect to perform .find on one collection to find any of the types, since a getAnimal query might return any type of animal.
   • I also end up defining defaults for all fields, even if they're not relevant to that type.
   • The __typename ends up determining what objects are compatible. I can no longer enforce a union here (e.g. Animal interface potentially has "Cat" | "Dog" | "Elephant") since the __typename determines how the type is matched and what fields must be present.
   • __typename can technically be undefined, but nullable only adds | null. I don't really want a default here.
   • Workaround: __typename: () => "Cat" as any, (or undefined as any will work too)

At this point it's a little easier to use .query<any, Variables>, but the types were otherwise helpful for developer experience and catching missing fields.