add typescript types, begin typescript test

test/types/models.test.ts

@@ -880,3 +880,25 @@ async function gh13999() {
     }
   }
 }
+
+function gh4727() {
+  const userSchema = new mongoose.Schema({
+    name: String
+  });
+  const companySchema = new mongoose.Schema({
+    name: String,
+    users: [{ ref: 'User', type: mongoose.Schema.Types.ObjectId }]
+  });
+
+  mongoose.model('UserTestHydrate', userSchema);
+  const Company = mongoose.model('CompanyTestHyrdrate', companySchema);
+
+  const users = [{ _id: new mongoose.Types.ObjectId(), name: 'Val' }];
+  const company = { _id: new mongoose.Types.ObjectId(), name: 'Booster', users: [users[0]] };
+
+  expectType<ReturnType<(typeof Company)['hydrate']> | null>(
+    Company.hydrate(company, null, { hydratedPopulatedDocs: true })
+  );
+
+
+}

types/models.d.ts

@@ -37,6 +37,11 @@ declare module 'mongoose' {
     skipValidation?: boolean;
   }
 
+  interface HydrateOptions {
+    setters?: boolean;
+    hydratedPopulatedDocs?: boolean;
+  }
+
   interface InsertManyOptions extends
     PopulateOption,
     SessionOption {
@@ -371,7 +376,7 @@ declare module 'mongoose' {
      * Shortcut for creating a new Document from existing raw data, pre-saved in the DB.
      * The document returned has no paths marked as modified initially.
      */
-    hydrate(obj: any, projection?: AnyObject, options?: { setters?: boolean }): THydratedDocumentType;
+    hydrate(obj: any, projection?: AnyObject, options?: HydrateOptions): THydratedDocumentType;
 
     /**
      * This function is responsible for building [indexes](https://www.mongodb.com/docs/manual/indexes/),