types(schematypes): use DocType for instance method this

test/types/schema.test.ts

@@ -1183,3 +1183,38 @@
   type InferredType = InferSchemaType<typeof schema>;
   expectType<bigint | undefined>(null as unknown as InferredType['num']);
 }
+
+function gh13800() {
+  interface IUser {
+    firstName: string;
+    lastName: string;
+    someOtherField: string;
+  }
+  interface IUserMethods {
+    fullName(): string;
+  }
+  type UserModel = Model<IUser, {}, IUserMethods>;
+
+  // Typed Schema
+  const schema = new Schema<IUser, UserModel, IUserMethods>({
+    firstName: { type: String, required: true },
+    lastName: { type: String, required: true }
+  });
+  schema.method('fullName', function fullName() {
+    expectType<string>(this.firstName);
+    expectType<string>(this.lastName);
+    expectType<string>(this.someOtherField);
+    expectType<IUserMethods['fullName']>(this.fullName);
+  });
+
+  // Auto Typed Schema
+  const autoTypedSchema = new Schema({
+    firstName: { type: String, required: true },
+    lastName: { type: String, required: true }
+  });
+  autoTypedSchema.method('fullName', function fullName() {
+    expectType<string>(this.firstName);
+    expectType<string>(this.lastName);
+    expectError<string>(this.someOtherField);
+  });
+}

types/index.d.ts

@@ -293,7 +293,7 @@ declare module 'mongoose' {
     loadClass(model: Function, onlyVirtuals?: boolean): this;
 
     /** Adds an instance method to documents constructed from Models compiled from this schema. */
-    method<Context = any>(name: string, fn: (this: Context, ...args: any[]) => any, opts?: any): this;
+    method<Context = THydratedDocumentType>(name: string, fn: (this: Context, ...args: any[]) => any, opts?: any): this;
     method(obj: Partial<TInstanceMethods>): this;
 
     /** Object of currently defined methods on this schema. */