(schema validation): Warn against referential action choices that do not make sense

[tomhoule]

Problem

With the referentialActions preview feature, some combinations of relations and relational actions make more sense than others. For example, SetNull on a required relation will (should) always fail at runtime. Restrict on a nullable relation is possible, but tricky to justify.

Suggested solution

The schema validation should display a warning when one of these actions does not make sense, through the editor extension.

Alternatives

We could make this a validation error, but since the databases we support accept these combinations, it would mean that introspection would result in broken schemas.

Additional context

• Preview feature feedback issue: Preview feature feedback: Referential Actions (referentialActions) #7816

[Sytten]

I don't agree with:

Restrict on a nullable relation is possible, but tricky to justify.

When testing the feature yesterday I had a lot of nullable restrict for one client. The reasoning is yes it might be optional, but once it is set it must not be removed. A simple example is a tax applied to an item.

[tomhoule]

Addition: set default on MySQL.