Fragments on interfaces should have exposed generated types

[twavv]

Currently, creating a fragment on an interface type generally results in that type being "inlined" into the Pydantic models where they're used.

# Schema
interface Actor {
  id: ID!
  username: String!
  # ...
}
type User implements Actor { ... }
type Bot implements Actor { ... }

type Query {
  actor(id: ID!): Actor
}


# Queries
fragment ActorInfo on Actor {
  id
  ... on User { ... }
  ... on Bot { ... }
}

query GetActor($id: ID!) {
  actor(id: $id) { ...ActorInfo }
}

results in the generated code akin to

class GetActor(BaseModel):
    actor: Optional[GetActorActorActor | GetActorActorUser | GetActorActorBot]

(i've omitted some of the less interesting bits)

It would be nice if this was exposed directly in the generated fragments file that way it can be referenced in other places in the code that consumes the GraphQL API.

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The hardest thing with this approach is that it's hard to combine with Python inheritance I think.

For example, if the query above didn't modify the fragment but added username to the result set in the query, it wouldn't be possible to inherit from the union to extend the type with the additional username field. This library doesn't currently use inheritance for that anyway,,, but,,, yeah.

Just ran into this issue and though I could post this for posterity at least.

[rafalp]

I agree with the usecase for this, but I am marking this as "decision needed" because its something that I have no idea currently how we can achieve in the codegen.