[mongoose] Define lean() result type (#42652)

types/mongoose/index.d.ts

@@ -75,6 +75,12 @@ declare module "mongoose" {
   import stream = require('stream');
   import mongoose = require('mongoose');
 
+  // We can use TypeScript Omit once minimum required TypeScript Version is above 3.5
+  type Omit<T, K> = Pick<T, Exclude<keyof T, K>>;
+
+  /* Helper type to extract a definition type from a Document type */
+  type DocumentDefinition<T> = Omit<T, keyof Document> & { _id: mongodb.ObjectId };
+
   /**
    * Gets and optionally overwrites the function used to pluralize collection names
    * @param fn function to use for pluralization of collection names
@@ -2042,7 +2048,7 @@ declare module "mongoose" {
      * getters/setters or other Mongoose magic applied.
      * @param {Boolean|Object} bool defaults to true
      */
-    lean<P = any>(bool?: boolean | object): Query<T extends Array<any> ? P[] : (P | null)> & QueryHelpers;
+    lean<P = DocumentDefinition<DocType>>(bool?: boolean | object): Query<T extends Array<any> ? P[] : (P | null)> & QueryHelpers;
 
     /** Specifies the maximum number of documents the query will return. Cannot be used with distinct() */
     limit(val: number): this;

types/mongoose/mongoose-tests.ts

@@ -1302,6 +1302,29 @@ query.lean() // true
 query.lean(false)
 query.lean({})
 
+interface Location1 extends mongoose.Document {
+    name: string;
+    address: string;
+    rating: number;
+    reviews: any[];
+};
+var locQuery = <mongoose.DocumentQuery<Location1, Location1>>{};
+async function leanTests() {
+    var location = await locQuery.lean().exec();
+    if (location) {
+        // $ExpectType ObjectId
+        location._id;
+        // $ExpectType string
+        location.name;
+        // $ExpectType number
+        location.rating;
+        // $ExpectError
+        location.unknown;
+        // $ExpectError
+        location.save();
+    }
+}
+
 /*
  * section schema/array.js
  * http://mongoosejs.com/docs/api.html#schema-array-js
@@ -1829,12 +1852,6 @@ interface ModelUser {
   name: string;
   abctest: string;
 }
-MongoModel.findOne({ type: 'iphone' }).select('name').lean().exec()
-.then(function(doc: ModelUser) {
-  doc._id;
-  doc.name;
-  doc.abctest;
-});
 MongoModel.findOneAndRemove({}, {}, cb);
 MongoModel.findOneAndRemove({}, {});
 MongoModel.findOneAndRemove({}, cb);
@@ -2026,6 +2043,20 @@ LocModel.findOne({}, function (err, doc) {
     doc.openingTimes;
   }
 });
+LocModel
+    .findOne({ name: 'foo' })
+    .lean()
+    .exec()
+    .then(function(doc) {
+        if (doc) {
+            // $ExpectType ObjectId
+            doc._id;
+            // $ExpectType string
+            doc.name;
+            // $ExpectError
+            doc.unknown;
+        }
+    });
 LocModel.findOneAndRemove()
   .exec(function (err, location) {
     if (location) {