fix(NODE-3454): projection types are too narrow

src/collection.ts

@@ -676,27 +676,27 @@ export class Collection<TSchema extends Document = Document> {
   findOne(callback: Callback<TSchema | undefined>): void;
   findOne(filter: Filter<TSchema>): Promise<TSchema | undefined>;
   findOne(filter: Filter<TSchema>, callback: Callback<TSchema | undefined>): void;
-  findOne(filter: Filter<TSchema>, options: FindOptions<TSchema>): Promise<TSchema | undefined>;
+  findOne(filter: Filter<TSchema>, options: FindOptions): Promise<TSchema | undefined>;
   findOne(
     filter: Filter<TSchema>,
-    options: FindOptions<TSchema>,
+    options: FindOptions,
     callback: Callback<TSchema | undefined>
   ): void;
 
   // allow an override of the schema.
   findOne<T = TSchema>(): Promise<T | undefined>;
   findOne<T = TSchema>(callback: Callback<T | undefined>): void;
   findOne<T = TSchema>(filter: Filter<T>): Promise<T | undefined>;
-  findOne<T = TSchema>(filter: Filter<T>, options?: FindOptions<T>): Promise<T | undefined>;
+  findOne<T = TSchema>(filter: Filter<T>, options?: FindOptions): Promise<T | undefined>;
   findOne<T = TSchema>(
     filter: Filter<T>,
-    options?: FindOptions<T>,
+    options?: FindOptions,
     callback?: Callback<T | undefined>
   ): void;
 
   findOne(
     filter?: Filter<TSchema> | Callback<TSchema | undefined>,
-    options?: FindOptions<TSchema> | Callback<TSchema | undefined>,
+    options?: FindOptions | Callback<TSchema | undefined>,
     callback?: Callback<TSchema>
   ): Promise<TSchema | undefined> | void {
     if (callback != null && typeof callback !== 'function') {
@@ -728,9 +728,9 @@ export class Collection<TSchema extends Document = Document> {
    * @param filter - The filter predicate. If unspecified, then all documents in the collection will match the predicate
    */
   find(): FindCursor<TSchema>;
-  find(filter: Filter<TSchema>, options?: FindOptions<TSchema>): FindCursor<TSchema>;
-  find<T = TSchema>(filter: Filter<T>, options?: FindOptions<T>): FindCursor<T>;
-  find(filter?: Filter<TSchema>, options?: FindOptions<TSchema>): FindCursor<TSchema> {
+  find(filter: Filter<TSchema>, options?: FindOptions): FindCursor<TSchema>;
+  find<T = TSchema>(filter: Filter<T>, options?: FindOptions): FindCursor<T>;
+  find(filter?: Filter<TSchema>, options?: FindOptions): FindCursor<TSchema> {
     if (arguments.length > 2) {
       throw new MongoInvalidArgumentError(
         'Method "collection.find()" accepts at most two arguments'

src/cursor/aggregation_cursor.ts

@@ -9,7 +9,6 @@ import type { Callback, MongoDBNamespace } from '../utils';
 import type { ClientSession } from '../sessions';
 import type { AbstractCursorOptions } from './abstract_cursor';
 import type { ExplainVerbosityLike } from '../explain';
-import type { Projection } from '../mongo_types';
 
 /** @public */
 export interface AggregationCursorOptions extends AbstractCursorOptions, AggregateOptions {}
@@ -135,22 +134,43 @@ export class AggregationCursor<TSchema = Document> extends AbstractCursor<TSchem
    * In order to strictly type this function you must provide an interface
    * that represents the effect of your projection on the result documents.
    *
-   * **NOTE:** adding a projection changes the return type of the iteration of this cursor,
+   * By default chaining a projection to your cursor changes the returned type to the generic {@link Document} type.
+   * You should specify a parameterized type to have assertions on your final results.
+   *
+   * @example
+   * ```typescript
+   * // Best way
+   * const docs: AggregationCursor<{ a: number }> = cursor.project<{ a: number }>({ _id: 0, a: true });
+   * // Flexible way
+   * const docs: AggregationCursor<Document> = cursor.project({ _id: 0, a: true });
+   * ```
+   *
+   * @remarks
+   * In order to strictly type this function you must provide an interface
+   * that represents the effect of your projection on the result documents.
+   *
+   * Adding a projection changes the return type of the iteration of this cursor,
    * it **does not** return a new instance of a cursor. This means when calling project,
    * you should always assign the result to a new variable. Take note of the following example:
    *
    * @example
    * ```typescript
    * const cursor: AggregationCursor<{ a: number; b: string }> = coll.aggregate([]);
-   * const projectCursor = cursor.project<{ a: number }>({ a: true });
+   * const projectCursor = cursor.project<{ a: number }>({ _id: 0, a: true });
    * const aPropOnlyArray: {a: number}[] = await projectCursor.toArray();
+   *
+   * // or always use chaining and save the final cursor
+   *
+   * const cursor = coll.aggregate().project<{ a: string }>({
+   *   _id: 0,
+   *   a: { $convert: { input: '$a', to: 'string' }
+   * }});
    * ```
    */
-  project<T = TSchema>($project: Projection<T>): AggregationCursor<T>;
-  project($project: Document): this {
+  project<T extends Document = Document>($project: Document): AggregationCursor<T> {
     assertUninitialized(this);
     this[kPipeline].push({ $project });
-    return this;
+    return (this as unknown) as AggregationCursor<T>;
   }
 
   /** Add a lookup stage to the aggregation pipeline */

src/cursor/find_cursor.ts

@@ -12,7 +12,6 @@ import type { ClientSession } from '../sessions';
 import { formatSort, Sort, SortDirection } from '../sort';
 import type { Callback, MongoDBNamespace } from '../utils';
 import { AbstractCursor, assertUninitialized } from './abstract_cursor';
-import type { Projection } from '../mongo_types';
 
 /** @internal */
 const kFilter = Symbol('filter');
@@ -344,22 +343,42 @@ export class FindCursor<TSchema = Document> extends AbstractCursor<TSchema> {
    * In order to strictly type this function you must provide an interface
    * that represents the effect of your projection on the result documents.
    *
-   * **NOTE:** adding a projection changes the return type of the iteration of this cursor,
+   * By default chaining a projection to your cursor changes the returned type to the generic
+   * {@link Document} type.
+   * You should specify a parameterized type to have assertions on your final results.
+   *
+   * @example
+   * ```typescript
+   * // Best way
+   * const docs: FindCursor<{ a: number }> = cursor.project<{ a: number }>({ _id: 0, a: true });
+   * // Flexible way
+   * const docs: FindCursor<Document> = cursor.project({ _id: 0, a: true });
+   * ```
+   *
+   * @remarks
+   *
+   * Adding a projection changes the return type of the iteration of this cursor,
    * it **does not** return a new instance of a cursor. This means when calling project,
    * you should always assign the result to a new variable. Take note of the following example:
    *
    * @example
    * ```typescript
    * const cursor: FindCursor<{ a: number; b: string }> = coll.find();
-   * const projectCursor = cursor.project<{ a: number }>({ a: true });
+   * const projectCursor = cursor.project<{ a: number }>({ _id: 0, a: true });
    * const aPropOnlyArray: {a: number}[] = await projectCursor.toArray();
+   *
+   * // or always use chaining and save the final cursor
+   *
+   * const cursor = coll.find().project<{ a: string }>({
+   *   _id: 0,
+   *   a: { $convert: { input: '$a', to: 'string' }
+   * }});
    * ```
    */
-  project<T = TSchema>(value: Projection<T>): FindCursor<T>;
-  project(value: Projection<TSchema>): this {
+  project<T extends Document = Document>(value: Document): FindCursor<T> {
     assertUninitialized(this);
     this[kBuiltOptions].projection = value;
-    return this;
+    return (this as unknown) as FindCursor<T>;
   }
 
   /**

src/mongo_types.ts

@@ -169,20 +169,19 @@ export type BSONType = typeof BSONType[keyof typeof BSONType];
 /** @public */
 export type BSONTypeAlias = keyof typeof BSONType;
 
-/** @public */
-export interface ProjectionOperators extends Document {
-  $elemMatch?: Document;
-  $slice?: number | [number, number];
-  $meta?: string;
-  /** @deprecated Since MongoDB 3.2, Use FindCursor#max */
-  $max?: any;
-}
+/**
+ * @public
+ * Projection is flexible to permit the wide array of aggregation operators
+ * @deprecated since v4.1.0: Since projections support all aggregation operations we have no plans to narrow this type further
+ */
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+export type Projection<TSchema extends Document = Document> = Document;
 
-/** @public */
-export type Projection<TSchema> = {
-  [Key in keyof TSchema]?: ProjectionOperators | 0 | 1 | boolean;
-} &
-  Partial<Record<string, ProjectionOperators | 0 | 1 | boolean>>;
+/**
+ * @public
+ * @deprecated since v4.1.0: Since projections support all aggregation operations we have no plans to narrow this type further
+ */
+export type ProjectionOperators = Document;
 
 /** @public */
 export type IsAny<Type, ResultIfAny, ResultIfNotAny> = true extends false & Type

src/operations/find.ts

@@ -15,16 +15,19 @@ import { Sort, formatSort } from '../sort';
 import { isSharded } from '../cmap/wire_protocol/shared';
 import { ReadConcern } from '../read_concern';
 import type { ClientSession } from '../sessions';
-import type { Projection } from '../mongo_types';
 
-/** @public */
-export interface FindOptions<TSchema = Document> extends CommandOperationOptions {
+/**
+ * @public
+ * @typeParam TSchema - Unused schema definition, deprecated usage, only specify `FindOptions` with no generic
+ */
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+export interface FindOptions<TSchema extends Document = Document> extends CommandOperationOptions {
   /** Sets the limit of documents returned in the query. */
   limit?: number;
   /** Set to sort the documents coming back from the query. Array of indexes, `[['a', 1]]` etc. */
   sort?: Sort;
   /** The fields to return in the query. Object of fields to either include or exclude (one of, not both), `{'a':1, 'b': 1}` **or** `{'a': 0, 'b': 0}` */
-  projection?: Projection<TSchema>;
+  projection?: Document;
   /** Set to skip N documents ahead in your query (useful for pagination). */
   skip?: number;
   /** Tell the query to use specific indexes in the query. Object of indexes to use, `{'_id':1}` */

test/types/community/collection/findX.test-d.ts

@@ -1,5 +1,6 @@
-import { expectNotType, expectType } from 'tsd';
+import { expectAssignable, expectNotType, expectType } from 'tsd';
 import { FindCursor, FindOptions, MongoClient, Document } from '../../../../src';
+import type { Projection, ProjectionOperators } from '../../../../src';
 import type { PropExists } from '../../utility_types';
 
 // collection.findX tests
@@ -35,7 +36,7 @@ await collectionT.findOne(
   }
 );
 
-const optionsWithComplexProjection: FindOptions<TestModel> = {
+const optionsWithComplexProjection: FindOptions = {
   projection: {
     stringField: { $meta: 'textScore' },
     fruitTags: { $min: 'fruitTags' },
@@ -120,14 +121,15 @@ function printCar(car: Car | undefined) {
   console.log(car ? `A car of ${car.make} make` : 'No car');
 }
 
-const options: FindOptions<Car> = {};
-const optionsWithProjection: FindOptions<Car> = {
+const options: FindOptions = {};
+const optionsWithProjection: FindOptions = {
   projection: {
     make: 1
   }
 };
 
-expectNotType<FindOptions<Car>>({
+// this is changed in NODE-3454 to be the opposite test since Projection is flexible now
+expectAssignable<FindOptions>({
   projection: {
     make: 'invalid'
   }
@@ -190,3 +192,9 @@ expectType<FindCursor<{ color: { $in: number } }>>(colorCollection.find({ color:
 // When you use the override, $in doesn't permit readonly
 colorCollection.find<{ color: string }>({ color: { $in: colorsFreeze } });
 colorCollection.find<{ color: string }>({ color: { $in: ['regularArray'] } });
+// This is a regression test that we don't remove the unused generic in FindOptions
+const findOptions: FindOptions<{ a: number }> = {};
+expectType<FindOptions>(findOptions);
+// This is just to check that we still export these type symbols
+expectAssignable<Projection>({});
+expectAssignable<ProjectionOperators>({});

test/types/community/cursor.test-d.ts

@@ -1,6 +1,6 @@
 import type { Readable } from 'stream';
 import { expectNotType, expectType } from 'tsd';
-import { FindCursor, MongoClient } from '../../../src/index';
+import { FindCursor, MongoClient, Db, Document } from '../../../src/index';
 
 // TODO(NODE-3346): Improve these tests to use expect assertions more
 
@@ -22,7 +22,7 @@ const cursor = collection
   .min({ age: 18 })
   .maxAwaitTimeMS(1)
   .maxTimeMS(1)
-  .project({})
+  // .project({}) -> projections removes the types from the returned documents
   .returnKey(true)
   .showRecordId(true)
   .skip(1)
@@ -31,6 +31,7 @@ const cursor = collection
 
 expectType<FindCursor<{ foo: number }>>(cursor);
 expectType<Readable>(cursor.stream());
+expectType<FindCursor<Document>>(cursor.project({}));
 
 collection.find().project({});
 collection.find().project({ notExistingField: 1 });
@@ -74,13 +75,13 @@ expectNotType<{ age: number }[]>(await typedCollection.find().project({ name: 1
 expectType<{ notExistingField: unknown }[]>(
   await typedCollection.find().project({ notExistingField: 1 }).toArray()
 );
-expectNotType<TypedDoc[]>(await typedCollection.find().project({ notExistingField: 1 }).toArray());
+expectType<TypedDoc[]>(await typedCollection.find().project({ notExistingField: 1 }).toArray());
 
 // Projection operator
 expectType<{ listOfNumbers: number[] }[]>(
   await typedCollection
     .find()
-    .project({ listOfNumbers: { $slice: [0, 4] } })
+    .project<{ listOfNumbers: number[] }>({ listOfNumbers: { $slice: [0, 4] } })
     .toArray()
 );
 
@@ -98,3 +99,57 @@ void async function () {
     expectType<number>(item.foo);
   }
 };
+
+interface InternalMeme {
+  _id: string;
+  owner: string;
+  receiver: string;
+  createdAt: Date;
+  expiredAt: Date;
+  description: string;
+  likes: string;
+  private: string;
+  replyTo: string;
+  imageUrl: string;
+}
+
+interface PublicMeme {
+  myId: string;
+  owner: string;
+  likes: number;
+  someRandomProp: boolean; // Projection makes no enforcement on anything
+  // the convenience parameter project<X>() allows you to define a return type,
+  // otherwise projections returns generic document
+}
+
+const publicMemeProjection = {
+  myId: { $toString: '$_id' },
+  owner: { $toString: '$owner' },
+  receiver: { $toString: '$receiver' },
+  likes: '$totalLikes' // <== (NODE-3454) cause of TS2345 error: Argument of type T is not assignable to parameter of type U
+};
+const memeCollection = new Db(new MongoClient(''), '').collection<InternalMeme>('memes');
+
+expectType<PublicMeme[]>(
+  await memeCollection
+    .find({ _id: { $in: [] } })
+    .project<PublicMeme>(publicMemeProjection) // <==
+    .toArray()
+);
+
+// Returns generic document when no override given
+expectNotType<InternalMeme[]>(
+  await memeCollection
+    .find({ _id: { $in: [] } })
+    .project(publicMemeProjection)
+    .toArray()
+);
+
+// Returns generic document when there is no schema
+expectType<Document[]>(
+  await new Db(new MongoClient(''), '')
+    .collection('memes')
+    .find({ _id: { $in: [] } })
+    .project(publicMemeProjection)
+    .toArray()
+);