Tolerate missing identifier when applySnapshot to map

[davetapley]

I'm writing an API, and I'd love to just applySnapshot my API's response in the front end.

However for a map getSnapshot returns an object where the identifier is both the key in outer object and a key in the inner object (the value), e.g.:

export const StageData = model({
  unit: frozen<LengthUnits>(),
  latest: Sample,
  max: map(Period),
});

// create instance and toSnapshot:
    {
      unit: 'ft',
      latest: { ts: '2025-01-16T00:09:47.392Z', value: 1 },
      max: { P1D: { period: 'P1D', sample: [Object] } }
    }

(note P1D is present twice)

---

If I omit period: 'P1D' in the nested object and applySnapshot I get:

[mobx-state-tree] Error while converting `{"unit":"in","latest":{"ts":"2025-01-16T01:17:43.272Z","value":1},"max":{"P1D":{"sample":{"ts":"2025-01-16T01:17:43.272Z","value":1}}}}` to `AnonymousModel`:

        at path "/max/P1D/period" value `undefined` is not assignable to type: `identifier` (Value is not a valid identifier, expected a string).

As a workaround I can include the duplicate period: 'P1D' in my API response, but:

Since each object in a map only has one identifier and it is by definition the key of map object, could that be inferred?

[coolsoftwaretyler]

Hey @davetapley - thanks for the write up here. I don't think we can make this work by default with all types.map instances, because this is not always the case:

Since each object in a map only has one identifier and it is by definition the key of map object, could that be inferred?

This is only true if you are using a model type that has an identifier. But for some data, you may not use types.identifier at all, so we can't guarantee being able to infer things.

Can you share with me more about an ideal solution? What do you want the snapshots to look like instead? I am not sure if I can really picture how we would snapshot a map of objects with IDs, but not include the keys in the map or the IDs in the objects, but maybe I'm misunderstanding.

[davetapley]

Oh! I completely misinterpreted the docs.
I'd assumed a types.identifier was required, and when I saw it echoed n the getSnapshot output I further assumed it was an internal design decision.

I was about to suggest clarifying docs, but now I see:

If the type stored in a map has an identifier

So, I should just have RTFM 😆

[coolsoftwaretyler]

Haha all good! They are not always the clearest docs in the world. Glad this cleared things up for ya.