Model.exists result _id is the document's type

[iammola]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.3

Node.js version

18.4.0

MongoDB server version

5.0

Description

Using Model.exists to check if a document exists with the specified query is meant to return the Document's _id if it exists and null otherwise.

But I get a TS error when accessing the _id because it's not the expected value type.

When I read the types to understand the error in these lines,

mongoose/types/models.d.ts

Lines 308 to 313 in db1ba6e

	/**
	* Returns a document with its `_id` if at least one document exists in the database that matches
	* the given `filter`, and `null` otherwise.
	*/
	exists(filter: FilterQuery<T>, callback: Callback<Pick<Document<T>, '_id'> | null>): QueryWithHelpers<Pick<Document<T>, '_id'> | null, HydratedDocument<T, TMethodsAndOverrides, TVirtuals>, TQueryHelpers, T>;
	exists(filter: FilterQuery<T>): QueryWithHelpers<Pick<Document<T>, '_id'> | null, HydratedDocument<T, TMethodsAndOverrides, TVirtuals>, TQueryHelpers, T>;

The first parameter in the Document class is expected to be _id's type. But the T variable that's passed is the Schema's resolved type.

mongoose/types/document.d.ts

Lines 13 to 23 in db1ba6e

	/**
	* Generic types for Document:
	* * T - the type of _id
	* * TQueryHelpers - Object with any helpers that should be mixed into the Query type
	* * DocType - the type of the actual Document created
	*/
	class Document<T = any, TQueryHelpers = any, DocType = any> {
	constructor(doc?: any);
	/** This documents _id. */
	_id?: T;

---

From the documatation,

Under the hood, MyModel.exists({ answer: 42 }) is equivalent to MyModel.findOne({ answer: 42 }).select({ _id: 1 }).lean()

So the RequireOnlyTypedId type could be better to use as the Model.exists return type because the _id property always exists and the returned document is always a POJO unless it's not found then null.

mongoose/types/index.d.ts

Lines 117 to 119 in db1ba6e

	export type RequireOnlyTypedId<T> = T extends { _id?: infer U; }
	? Required<{ _id: U }>
	: { _id: Types.ObjectId };

Then the method definition would be something like,

exists(filter: FilterQuery<T>, callback: Callback<RequireOnlyTypedId<T> | null>): QueryWithHelpers<RequireOnlyTypedId<T> | null, HydratedDocument<T, TMethodsAndOverrides, TVirtuals>, TQueryHelpers, T>; 
exists(filter: FilterQuery<T>): QueryWithHelpers<RequireOnlyTypedId<T> | null, HydratedDocument<T, TMethodsAndOverrides, TVirtuals>, TQueryHelpers, T>; 

Steps to Reproduce

const schema = new mongoose.Schema({ name: String });
const Model = mongoose.model('Model', schema);

Model.exists({ name: "Hey!" }).then(doc => {
  if (doc != null) {
    const ID: mongoose.Types.ObjectId = doc._id;
    // Type '{ name?: string | undefined; } | undefined' is not assignable to type 'ObjectId'.
    // Type 'undefined' is not assignable to type 'ObjectId'.ts(2322)
  }
});

Expected Behavior

For the _id property to be the Document's _id type not the entire document type.

[IslandRhythms]

import * as mongoose from 'mongoose';

// 1. Create an interface representing a document in MongoDB.
interface IUser {
  name: string;
  email: string;
  avatar?: string;
}

// 2. Create a Schema corresponding to the document interface.
const userSchema = new mongoose.Schema<IUser>({
  name: { type: String, required: true },
  email: { type: String, required: true },
  avatar: String
});

// 3. Create a Model.
const User = mongoose.model<IUser>('User', userSchema);

run().catch(err => console.log(err));

async function run() {
  // 4. Connect to MongoDB
  await mongoose.connect('mongodb://localhost:27017/test');
  await mongoose.connection.dropDatabase();

  const user = new User({
    name: 'Bill',
    email: 'bill@initech.com',
    avatar: 'https://i.imgur.com/dM7Thhn.png'
  });
  await user.save();

  console.log(user.email); // 'bill@initech.com'
  User.exists({ name: 'Bill' }).then(doc => {
    if(doc != null) {
      const ID : mongoose.Types.ObjectId = doc._id;
    }
  });
}