Given the type 'T & Document', can't type the object returned by .toObject() as T

[adriano-di-giovanni]

I don't know if it is a bug. It seems to be so. I can tell you it worked with v5.10.11. It doesn't work anymore with v5.11.16

Given this piece code

function transformOne<T>(document: T & Document): T {
  return document.toObject();
}

the compiler complaints as follows

Type 'Pick<Pick<_LeanDocument<T & Document<any>>, "_id" | "__v" | "id" | Exclude<keyof T, "$ignore" | "$isDefault" | "$isDeleted" | "$isEmpty" | "$isValid" | ... 46 more ... | "$isSingleNested">>, Exclude<...> | ... 2 more ... | Exclude<...>>' is not assignable to type 'T'.
  'T' could be instantiated with an arbitrary type which could be unrelated to 'Pick<Pick<_LeanDocument<T & Document<any>>, "_id" | "__v" | "id" | Exclude<keyof T, "$ignore" | "$isDefault" | "$isDeleted" | "$isEmpty" | "$isValid" | ... 46 more ... | "$isSingleNested">>, Exclude<...> | ... 2 more ... | Exclude<...>>'.ts(2322)

My tsconfig.json is as follows

{
  "compilerOptions": {
    "module": "commonjs",
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "allowSyntheticDefaultImports": true,
    "target": "es2017",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true
  }
}

[vkarpov15]

This is because @types/mongoose returned any from toObject(), which is not necessarily correct. In v5.11.x we make toObject() infer the type the returned object should have based on the schema, but that seems to cause problems for the design pattern you mentioned, even though T should be equivalent to LeanDocument<T & Document> in the above example.

We'll add a generic parameter to toObject() to allow you to overwrite the returned type, like this: return document.toObject<T>();