This works, but the result of the query (starting from an object id) varies. What can a static type system do under these circumstances? It seems like it might as well be a dynamic field access?

The result of a database query may have an immutable type, but it can also become stale, like an invalid cache entry. That's a possible bug. Perhaps there's some trick that would allow us to treat a live object as unchanging, with a fixed type and shape, but only within a limited scope in which we can prove it won't mutate?

For example, in a single-threaded language, it seems like we should be able to do a runtime check to get a more specific type and use it until the next await. The runtime check only needs to be done once in that scope.