Add null to select override return type for findOne

[sderrow]

Summary

Normally, a findOne or findById returns DocType | null. When using Query.prototype.select, we can provide a RawDocTypeOverride in order to define the true return type of the projected document. Currently, this makes findOne and find queries return RawDocTypeOverride directly, but in order to be consistent, it should return RawDocTypeOverride | null.

This is consistent with how you don't currently have to specify the array version for find; that method automatically returns the array version of RawDocTypeOverride. Therefore, select on findOne should return the nullable version of RawDocTypeOverride automatically.

Examples

import mongoose from "mongoose";

export const selectReturnType = async (): Promise<void> => {
  type Test = {
    _id: mongoose.Types.ObjectId;

    prop: string;
    another: string;

    createdAt: number;
    updatedAt: number;
  };

  const schema = new mongoose.Schema<Test>({
    prop: { type: String },
    another: {type: String },
    createdAt: { type: Number },
    updatedAt: { type: Number },
  });

  type TestDocument = mongoose.HydratedDocument<Test>;
  type TestModel = mongoose.Model<Test, {}, {}, {}, TestDocument>;

  type SlimTest = Pick<Test, "_id" | "prop">;
  type SlimTestDocument = mongoose.HydratedDocument<SlimTest>;

  const M = mongoose.model<Test, TestModel>("Test", schema);

  const myDocs = await M.find({}).exec();
  const myDoc = await M.findOne({}).exec();

  const myProjections = await M.find({}).select<SlimTest>({ prop: 1 }).exec();
  const myProjection = await M.findOne({}).select<SlimTest>({ prop: 1 }).exec();

  /*
  CURRENTLY:
    myDocs is of type TestDocument[]
    myDoc is of type TestDocument | null
    myProjections is of type SlimTestDocument[]
    myProjection is of type SlimTestDocument - THIS IS INCONSISTENT WITH THE NULLABLE RETURN TYPE OF myDoc (two lines above)

  WITH THIS CHANGE:
    [unchanged] myDocs is still of type TestDocument[]
    [unchanged] myDoc is still of type TestDocument | null
    [unchanged] myProjections is still of type SlimTestDocument[]
    [CHANGED] myProjection is now of type SlimTestDocument | null
  */
};

[vkarpov15]

LGTM. I'll make some lint fixes and add tests in master but code looks good.