cast TS error when types.array sub-type has reference

[Ayub-Begimkulov]

Bug report

• I've checked documentation and searched for existing issues
• I've made sure my project is based on the latest MST version
• Fork this code sandbox.

Sandbox link or minimal reproduction code

import { cast, Instance, types } from "mobx-state-tree";
import { v4 as uuid } from "uuid";

const Reference = types.model("Referense", {
  id: types.identifier,
  name: types.string
});

interface IReference extends Instance<typeof Reference> {}

const Entity = types.model("Entity", {
  id: types.identifier,
  name: types.string,
  ref: types.reference(Reference)
});

interface IEntity extends Instance<typeof Entity> {}

const RootModel = types
  .model("Store", {
    entities: types.array(Entity),
    references: types.array(Reference)
  })
  .actions((self) => ({
    setReferences(references: IReference[]) {
      // no references in the `Reference` model
      // so it works correctly
      self.references = cast(references);
    },
    setEntities(entities: IEntity[]) {
      // `Entity` model has a reference
      // so the cast isn't working
      self.entities = cast(entities);
    }
  }));

Describe the expected behavior

self.entities = cast(entities) should not have any typescript errors

Describe the observed behavior

TypeScript error:
Type '{ id: string; name: string; } & NonEmptyObject & IStateTreeNode<IReferenceType<IModelType<{ id: ISimpleType<string>; name: ISimpleType<string>; }, {}, _NotCustomized, _NotCustomized>>>' is not assignable to type 'ReferenceIdentifier'

[iChenLei]

castToSnapshot

castToSnapshot(snapshotOrInstance: I): Extract<I, IAnyStateTreeNode> extends never ? I : TypeOfValue["CreationType"]

In your reproduction example, replace cast with castToSnapshot:

setEntities(entities: IEntity[]) {
  // `Entity` model has a reference
  // so the cast isn't working
-  self.entities = cast(entities);
+  self.entities = castToSnapshot(entities);
}

looks like everything work fine!

MST castToSnapshot doc: https://mobx-state-tree.js.org/API/index#casttosnapshot

Defined in packages/mobx-state-tree/src/core/mst-operations.ts:941
Casts a node instance type to an snapshot type so it can be assigned to a type snapshot (e.g. to be used inside a create call). Note that this is just a cast for the type system, this is, it won't actually convert an instance to a snapshot, but just fool typescript into thinking so.

I am not a MST expert, so I don't know why it's work. Maybe Maintainer @jamonholmgren can explain explicitly.

[thegedge]

So this is tricky. self.entities is an IMSTArray, and you definitely can't assign a regular array to an IMSTArray, but MST technically does support this.

castToSnapshot works here, but it's not super helpful because it's actually just turning the right-hand side into never, which you can assign to whatever. I would probably just do this for now:

(self as any).entities = entities;

or you could do something like this:

type T = {
  get entities(): typeof self.entities
  set entities(v: (Instance<typeof RootModel> | SnapshotIn<typeof RootModel>)["entities"])
}

(self as T).entities = entities

I believe that's the "true" type of self in that scenario.

Once microsoft/TypeScript#43826 is resolved, we can generate the getter and setter types, like in T above :)