Add object to object array within mongodb model with graphql-compose-mongoose

I made a GraphQL API to retrieve and manipulate data in a mongo database. This data represents an author that contains a list of books as shown in Author.ts.

Author.ts:

import { Document, Model, model, Schema } from "mongoose";
import { Ibook } from "./book";

export interface Iauthor extends Document {
    name: String;
    books: Array<Ibook>;
  }
  
const AuthorSchema: Schema = new Schema({
    name: { type: String, required: true },
    books: { type: Array, required: true }
  });
  
export const Author: Model<Iauthor> = model('author', AuthorSchema);

Book.ts:

import {Document, Model, model, Schema} from "mongoose";

export interface Ibook extends Document {
    title: String;
    pages: Number;
  }
  
const BookSchema: Schema = new Schema({
    title: { type: String, required: true },
    pages: { type: Number, required: true }
  });
  
export const Book: Model<Ibook> = model('book', BookSchema);

I'm using query resolvers and mutation resolvers to read and manipulate these objects. I just migrated to graphql-compose-mongoose to generate the resolvers instead of programming them myself. I succeeded in migrating every query and mutation till I encountered my resolver "AssignBooktoAuthor". This basicly is a resolver that adds a book object to the authors books list. I simply do not know how to get it working with graphql-compose-mongoose.

composer.ts:

import { composeMongoose } from "graphql-compose-mongoose";
import { schemaComposer } from "graphql-compose";
import { Book } from "./models/book";
import { Author } from "./models/author";

const customizationOptions = {};
const BookTC = composeMongoose(Book, customizationOptions);
const AuthorTC = composeMongoose(Author, customizationOptions);

//in aparte file zetten prob
schemaComposer.Query.addFields({
  getAllBooks: BookTC.mongooseResolvers.findMany(),
  getBookById: BookTC.mongooseResolvers.findById(),
  getAllAuthors: AuthorTC.mongooseResolvers.findMany(),
  getAuthorById: AuthorTC.mongooseResolvers.findById(),
});

schemaComposer.Mutation.addFields({
  CreateBook: BookTC.mongooseResolvers.createOne(),
  UpdateBook: BookTC.mongooseResolvers.updateById(),
  DeleteBook: BookTC.mongooseResolvers.removeById(),
  CreateAuthor: AuthorTC.mongooseResolvers.createOne(),
  UpdateAuthor: AuthorTC.mongooseResolvers.updateById(),
  DeleteAuthor: AuthorTC.mongooseResolvers.removeById(),
  /**AssignBooktoAuthor: AuthorTC.mongooseResolvers.///What function does this??///**/
});

const graphqlSchema = schemaComposer.buildSchema();
export default graphqlSchema;

gql.ts (server file):

import { ApolloServer, gql } from "apollo-server-express";
import * as mongoose from "mongoose";
import * as express from "Express";
import * as dotenv from "dotenv";
import * as jwt from "jsonwebtoken";
import graphqlSchema from "./composer";

const app = express();
 
dotenv.config();

(async () => {
  mongoose.connect("mongodb+srv://xxxxxxx:[email protected]/myFirstDatabase?retryWrites=true&w=majority", { useUnifiedTopology: true , useNewUrlParser: true});

  const server = new ApolloServer({ 
    schema: graphqlSchema,
  });

  await server.start();

  //verifying token 
  app.use((req, res, next) => {
    const Token = req.header('auth-token');
    try{
      (Token && jwt.verify(Token, process.env.TOKEN_SECRET)) ? next() : console.log("mw: Unauthorized");
    }catch(err){
      console.log("mw:Unauthorized");
    }
  });

  server.applyMiddleware({ app });
  
  app.listen({ port: 4000 }, () =>
  console.log(`🚀 Server ready at http://localhost:4000${server.graphqlPath}`)
  );

})();

Since the support on graphql-compose-mongoose is not that big I hope there is someone who can help me with this problem!