[Feature Request] Nested routes in the RestApiHandler

[lsmith77]

This relates to this discussion thread on Discord.

Is your feature request related to a problem? Please describe.

RestApiHandler only supports flat routes (/post, /post/42). There is no way to express parent-scoped access in the URL structure. To fetch only a user's posts, clients must manually filter by parent ID — which is error-prone and produces URLs that don't reflect the data hierarchy.

** Describe the solution you'd like **
Add explicit nested route configuration to RestApiHandler that enables hierarchical URL patterns (/user/:id/post, /user/:id/post/:postId) for all five HTTP methods, with automatic parent scoping enforced server-side:

new RestApiHandler({
    schema: client.$schema,
    endpoint: 'http://localhost/api',
    nestedRoutes: {
        User: {
            Post: {
                relation: 'author',            // relation field on child pointing to parent
                requireOrphanProtection: true, // optional: enforce safe onDelete at startup
            },
        },
    },
});

Key behaviors:

• 404 if parent does not exist, or child does not belong to the requested parent.
• POST automatically binds the created child to the parent; returns 400 if the payload also sets that relation.
• Flat routes remain fully functional. Non-configured 3-segment paths fall through to fetchRelated unchanged.
• JSON:API self links in responses use the nested URL.
• requireOrphanProtection: true throws at startup if the configured relation does not have onDelete: Cascade, Restrict, or NoAction in the schema.
• All configuration errors (unknown models, missing relation fields, mismatched targets) are caught at construction time.

Describe alternatives you've considered

• Implicit route generation from schema relations (aka auto discovery of the route nesting) — rejected because it creates unexpected route surface area and is hard to audit. Explicit-only is safer.
• Client-side filtering via filter[authorId]=u1 on flat routes — works but is not server-enforced and doesn't produce hierarchical URLs.

Additional context

• modelNameMapping and externalIdMapping are both respected on nested routes.
• Multi-level nesting (/a/:id/b/:id/c) could be done later.

[ymc9]

Supported in 3.5.0