Support Single Table Subclasses

[stephenh]

In the past I've generally preferred class-per-table subclasses, but it might be neat to support single-table subclasses with a scheme like:

Designate a type column and determines the subclass in joist-codegen.json:

UserEvent: {
  fields: {
    type: {
      subclasses: {
        COMMENTED: { name: "CommentUserEvent", fields: ["comment"] },
        LIKED: { name: "LikeUserevent", fields: ["post", "author"] },
      }
    }
  }

Or maybe re-arranged slightly:

UserEvent: {
  subclassDiscriminator: "type",
  subclasses: {
    COMMENTED: { name: "CommentUserEvent", fields: ["comment"] },
    LIKED: { name: "LikeUserEvent", fields: ["post", "author"] },
  }

And then codegen would output:

• UserEventCodegen.ts as-is
• UserEvent.ts as-is
• CommentUserEventCodegen.ts, new, extends UserEvent, marks comment as required
• CommentUserEvent.ts, new, touched once, for subclass-specific business logic.

Questions:

• How do handle m2os to "only a subclass" i.e. only a CommentUserEvent. In CPTI (class per table), the m2o FK just points to the subclass column

[stephenh]

Implemented in #966