schema response per content type

[eladchen]

💬 Question here

Would you consider updating the schema response config to also support a response per the request "accept" header?

const schema = {
   response: {
      // Augment the current mapping (status code -> response schema) to support
      // status code -> accept header -> response schema
      200: {
         "application/vnd.bla.v1+json": ...
         "*/*": ...
      }
   }
}

fastify.get("/apples/:id", { schema } , (request, reply) => {
  switch (request.header["accept"]) {
    case "application/json":
       reply.send(...);
       break;
   
    case "application/vnd.bla.v1+json":
       reply.send(...);
       break;
  }
});

This would also be beneficial when fastify-swagger dynamically generates an OAS v3 definition.

fastify/fastify-swagger#117

[mcollina]

I think you can define separate routes using the constraint option: https://www.fastify.io/docs/latest/Reference/Routes/#constraints. However you are a bit on uncharted territories as I do not think anybody as tested this pattern. Officially supporting it would be great in both fastify and fastify-swagger.

Would you like to send a few PRs?

[eladchen]

Cool.

I'll try and find time to add this. Care to point me where in fastify source I should probably start?

[Eomm]

Here is where the response's schemas are compiled:

https://github.com/fastify/fastify/blob/224dc104260ad26f9baa0e46962f917963d41fe5/lib/validation.js#L11

[TimonVS]

I'm running into the same issue, especially the OpenAPI/Swagger part. I'm willing to contribute a solution for this, but constraints are dynamic, so I struggle to see how we'd be able to infer the correct response or request body for the OpenAPI specification. Any guidance would be very helpful!

import headerConstraintStrategy from "header-constraint-strategy";

const api = fastify({
    // ...
    constraints: {
        accept: headerConstraintStrategy("accept");
    },
});

api.post(
    `/assets`,
    {
        schema: Type.Object({ file: Type.String({ format: "binary" }) }),
        // While the constraint doesn't look dynamic, it only works because we registered the constraint in the `fastify` constructor. Using this to infer the accept header could work if we standardize the `accept` constraint?
        constraints: { accept: "multipart/form-data" },
    },
    async function(request, reply) {
        // ...
    },
);

api.post(
    `/assets`,
    {
        schema: Type.Object({ fileUrl: Type.String() }),
        constraints: { accept: "application/json" },
    },
    async function(request, reply) {
        // ...
    },
);

[Fdawgs]

Related issue: fastify/fastify#3902