After migration, I get a runtime error re Mongo/BSON _id serialization—this wasn't mentioned in the migration guide

[ghost]

Here the error message:

data.car._id.toHexString returned from 'load' in src/routes/cars/[carId]/+page.server.ts cannot be serialized as JSON
Error: data.car._id.toHexString returned from 'load' in src/routes/cars/[carId]/+page.server.ts cannot be serialized as JSON
    at check_serializability (file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:134:8)
    at check_serializability (file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:128:5)
    at check_serializability (file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:128:5)
    at check_serializability (file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:128:5)
    at load_server_data (file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:34:3)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async file:///carshop/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/src/runtime/server/page/index.js:163:13

and _id is a Mongo bson id, before all worked fine.

Is this related to #6143 and #6008 ?

What is the recommended migration way?

[infovore]

Hard to tell without seeing your actual +page.server.ts code - but yes, it's definitely related to the load function trying to return something that can't be serialized - from the looks of it, a function. (the function toHexString which is part of the id object. I don't know enough about Mongo/bson to be able to comment further without seeing the code, and that means it's hard to tell if #6008 would fix it, or if there's still a tweak needed.

[ghost]

to be able to comment further without seeing the code

I just return a result, a document, from MongoDB, nothing more:

...
if (result)
  return {
    car: result
  }
...

This result/document includes as a Mongo document always its id, a so called ObjectId with the key _id and more in case of foreign keys.

I assume I need now a serializer which supports ObjectId, e.g., superjson but how do I integrate it?

[infovore]

well - you could serialize it by hand, ie, rather than just returning the result, transforming it yourself, or running it through a serializer in +page.server.ts and then, on object load inside +page.svelte, unserializing. Not knowing enough about this particular Mongo object: do you need all the methods on it, or is the raw JSON representation enough? (The sticking point will probably be Dates if you have them).

Looking at the docs, that seems like it might be as straightforward as importing superjson and then using superjson.serialize(result), and then superjson.deserialize(car) on the page component.

But: that's almost certainly not going to serialize toHexString because it's a method, not a property. I don't know if you even need the methods or not. But you're not going to be able to pass an object with functions from back-end to front-end, and I don't know how you were before - unless your queries were only running on the client.

[infovore]

Honestly, as a starting point, try:

if(result)
  return {
    car: JSON.parse(JSON.serialize(result))
  }
}

which will send you back a pure JSON object representation of result, with methods stripped. That may be all you need. (It was on an issue I had related to this). If you were to say "that feels ugly", I'd agree, and there are likely better ways to do this. However, it may also reveal that you need "real" serialisation, owing to the usage of things like more complex fields.

[AlexRMU]

The fact is that _id is an object. You need to do lean or use json.