distinct return ts type is unclear

[avichai-sealights]

Do you want to request a feature or report a bug?
Bug(?)
What is the current behavior?
distinct returns an array of Query doc type

const schema = new Schema<IModelEntry>({
    field: String
})
const model: Model<IModelEntry> = this.connection.model<IModelEntry>("collection", schema);
const response = await model.distinct('field', {});

According to the docs: distinct returns the distinct values of the given field that match filter. So if field is a string, a string[] should return.
However response is of type IModelEntry[]

What is the expected behavior?
distinct returns array of string
What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
16.11, 5.13.14, 5.0.4

[IslandRhythms]

returns as type any[]

import * as mongoose from 'mongoose';

interface IModelEntry {
    field: string
}
const schema = new mongoose.Schema<IModelEntry>({
    field: String
})
const model: mongoose.Model<IModelEntry> = mongoose.model<IModelEntry>("collection", schema);

async function test() {
    const response = await model.distinct('field', {});
}

test();

[vkarpov15]

Confirmed that the return type of await model.distinct('field', {}); is any[]. Which is not incorrect - technically distinct() can return an array of mixed types for field because there's nothing stopping you from inserting a document with field = null or field = true from the mongo shell.

We'll add the ability to do await model.distinct<string>('field') to make distinct() resolve to string[] for v6.2.4 to work around this.

[kaulsh]

@vkarpov15 I was thinking the correct type of distinct() should be something like this:

distinct<DocKey extends string, Result = DocKey extends keyof TRawDocType ? TRawDocType[DocKey] : unknown>(
        field: DocKey,
        filter?: FilterQuery<TRawDocType>
    ): QueryWithHelpers<
      Array<Result>,
      THydratedDocumentType,
      TQueryHelpers,
      TRawDocType,
      'distinct'
    >;

This should behave more like mongodb does, i.e., if you give something that isn't a key on the document, you still get an empty array or something unknown[]. With my suggested type, if you do give the correct key from the schema, you'll get a typesafe result.

If you're okay with this type, I'd be happy to raise a PR with tests.

[vkarpov15]

@kaulshashank looks good as long as tests pass. Feel free to put in a PR, we'll review it. My only potential concern is that this change will be backwards breaking; but if it is, we can always merge into the 8.0 branch.