Declarative cache doesn't work with unions

[martinbonnin]

Description

Follow up from this slack discussion.

type User @typePolicy(keyFields: "id") {
  id 
  name
}

type UserError {
  code
}

union UserResult = UserError | User

type Query {
  getUser(id: ID!): UserResult
}

The UserResult type doesn't have keyfields and therefore this branch is not taken.

More generally, union result types are awkward to work with the cache. See also apollographql/apollo-kotlin#3827

[BoD]

Instead, we need to use the keyfields of obj["__typename"], but at the moment the codegen doesn't expose that info.

Note: same issue exists with interfaces if @typePolicy is applied to implementing types but not on the interface.

[BoD]

Resolved in #156.

There still is no solution for @fieldPolicy with unions if the ids are per type (as we can't guess the resulting concrete type).