Cannot return null for non-nullable field GraphQL Nestjs - mongodb

Question

im getting this error Cannot return null for non-nullable field CityBoundaries.city_name. from mongoDB, but if im using static json data, i get my response fine. Here's my code. Any help will be appreciated :)
city-boundries.model.ts
import { ObjectType, Field, ID, Int, Float } from '#nestjs/graphql';
import { Prop, Schema, SchemaFactory } from '#nestjs/mongoose';
import { Document } from 'mongoose';
import 'dotenv/config';
import { CityBoundiresFeatures } from 'src/types/city-boundries-features';
import mongoose from 'mongoose';
export type CityBoundriesDocument = CityBoundaries & Document;
#Schema()
#ObjectType()
export class CityBoundaries extends Document {
#Field(() => ID, { nullable: true })
_id: string | number;
#Prop()
#Field()
readonly city_admin_level: number;
#Prop({ required: true })
#Field()
readonly city_name: string;
#Prop({ required: true })
#Field(() => Float)
readonly city_id: number;
#Prop({ required: true })
#Field(() => Float)
readonly city_osm_id: number;
#Prop({
required: true,
type: mongoose.Schema.Types.ObjectId,
ref: 'CityBoundiresFeatures',
})
#Field(() => [CityBoundiresFeatures], { nullable: true })
features: CityBoundiresFeatures[];
#Prop({ required: true })
#Field({ nullable: true })
type: string;
}
export const CityBoundriesSchema = SchemaFactory.createForClass(CityBoundaries);
city-boundries.module.ts
import { Module } from '#nestjs/common';
import { MongooseModule } from '#nestjs/mongoose';
import {
CityBoundaries,
CityBoundriesSchema,
} from 'src/models/city-boundries.model';
import { CityBoundriesService } from 'src/services/city-boundries.service';
import { CityBoundriesResolvers } from 'src/resolvers/city-boundries.resolver';
import { Bop500_City, CitiesSchema } from 'src/models/cities.model';
import { CitiesService } from 'src/services/cities.service';
#Module({
imports: [
MongooseModule.forFeature([
{
name: CityBoundaries.name,
schema: CityBoundriesSchema,
},
{ name: Bop500_City.name, schema: CitiesSchema },
]),
],
providers: [CityBoundriesService, CityBoundriesResolvers, CitiesService],
})
export class CityBoundriesModule {}
cities.resolver.ts
import { Resolver, Query, Args, ResolveField, Parent } from '#nestjs/graphql';
import { Bop500_City } from 'src/models/cities.model';
import { CityBoundaries } from 'src/models/city-boundries.model';
import { CitiesService } from 'src/services/cities.service';
import { CityBoundriesService } from 'src/services/city-boundries.service';
#Resolver(() => Bop500_City)
export class CitiesResolvers {
constructor(
private readonly citiesService: CitiesService,
private readonly cityBoundariesService: CityBoundriesService,
) {}
#Query(() => [Bop500_City])
/**
* #returns list of all cities
*/
async getAllCities() {
return this.citiesService.findMany();
}
#Query(() => [Bop500_City], { nullable: true })
/**
* #param {string} value take city name as argument from user
* #returns detail of city
*/
async getCity(#Args('city') value: string) {
const res = await this.citiesService.findOneByName(value);
console.log('Response: ', res);
return res;
}
#ResolveField(() => CityBoundaries)
async getBoundries(#Parent() parent: Bop500_City) {
const res = await this.cityBoundariesService.findOneByCityName(
parent.city_name,
);
console.log('Response: ', res);
return res;
}
}
city-boundries.service.ts
import { Injectable } from '#nestjs/common';
import { Model } from 'mongoose';
import { InjectModel } from '#nestjs/mongoose';
import {
CityBoundaries,
CityBoundriesDocument,
} from 'src/models/city-boundries.model';
import * as cityBoundriesData from '../static/City_boundaries.json';
#Injectable()
export class CityBoundriesService {
constructor(
#InjectModel(CityBoundaries.name)
private cityBoundriesModel: Model<CityBoundriesDocument>,
) {}
/**
* #param {string} cityName take city name as argument from user
* #returns boundries of city
*/
async findOneByCityName(cityName: string) {
const res = await this.cityBoundriesModel.find({ city_name: cityName });
return res;
}
}
app.module.ts
import { Module } from '#nestjs/common';
import { GraphQLModule } from '#nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '#nestjs/apollo';
import { MongooseModule } from '#nestjs/mongoose';
import { CitiesModule } from './module/cities.module';
import 'dotenv/config';
import { CityBoundriesModule } from './module/city-boundries.module';
#Module({
imports: [
MongooseModule.forRoot(process.env.MONGODB_DB_URI, {
dbName: process.env.DB_NAME,
}),
GraphQLModule.forRoot<ApolloDriverConfig>({
driver: ApolloDriver,
autoSchemaFile: 'src/schema/schema.gql',
introspection: true,
playground: true,
}),
CitiesModule,
CityBoundriesModule,
],
})
export class AppModule {}