Add validation rule to ensure that @override(from:) references an existing source schema

[glen-84]

No description provided.

[Aenimus]

Would such validation inhibit migration? In a migration flow, I would expect the following (simplified) flow:

1. Define @override on the field taking responsibility (the source, i.e., the graph defining the directive).
2. Remove the field being overridden (the target, i.e., the graph defined under "from")
3. Remove the @override directive from the defining graph.

Now imagine you're slowly migrating a monolith into microservices. Your intention is to remove the monolith at the end of the process. You override the last remaining field defined in the monolith. When you remove that monolith from composition, there is an error because @override references a service that no longer exists.

This then becomes a bit of a chicken and an egg problem. You need to remove the @override directive, but you also want to get rid of the service that has been migrated.

[Aenimus]

I should have mentioned, the way WunderGraph currently handles this is by producing a warning rather than an error when a from argument refers to an unknown subgraph.

[dariuszkuc]

Yes this should be a warning.