nestjs ResolveField Cannot determine a GraphQL host type

could someone help me, problem with ResolveFeild Cannot determine a GraphQL host type

School model

import { Entity, Column, PrimaryGeneratedColumn, CreateDateColumn, OneToMany } from 'typeorm';
import { ObjectType, Field, ID } from '@nestjs/graphql';
import { Class } from '../class/class.model';

@Entity('school')
@ObjectType()
export class School {
    @Field(() => ID)
    @PrimaryGeneratedColumn()
    id: number;

    @Field()
    @Column()
    skz: string;

    @Field()
    @Column({ name : 'school_level' })
    schoolLevel: number;

    @Field(type => [Class], { nullable: true })
    @OneToMany(() => Class, classObj => classObj.school)
    classes: Class[];

    @Field()
    @Column()
    @CreateDateColumn()
    created_at: Date;
}

Class model

import { Entity, Column, PrimaryGeneratedColumn, CreateDateColumn, UpdateDateColumn, ManyToOne, OneToMany, JoinColumn } from 'typeorm';
import { School } from '../school/school.model';
import { Pupil } from '../pupil/pupil.model';
import { Field, ID, ObjectType } from '@nestjs/graphql';

@Entity('class')
@ObjectType()
export class Class {
    @Field(() => ID)
    @PrimaryGeneratedColumn()
    id: number;

    @Field()
    @Column()
    name: string;

    @Field(type => School)
    @ManyToOne(() => School, school => school.classes)
    @JoinColumn({name : 'school_id'})
    school: School;

    @Field()
    @Column()
    @CreateDateColumn()
    created_at: Date;
}

School resolver

import { Resolver, Query, ResolveField, Parent } from '@nestjs/graphql';
import { School } from './school.model';
import { Class } from './../class/class.model';
import { SchoolService } from './school.service';
import { ClassService } from '../class/class.service';
import { Inject } from '@nestjs/common';

@Resolver((of) => School)
export class SchoolResolver {
    constructor (@Inject(SchoolService) private schoolService: SchoolService,
        @Inject(ClassService) private classService: ClassService) {}

    @Query(() => [School])
    async schools() {
        return this.schoolService.getSchools()
    }

    @ResolveField(returns => [Class])
    async classes(@Parent() school: School) {
        const { id } = school;
        return this.classService.getClassesBySchool({ schoolId: id });
    }
}

(node:4572) UnhandledPromiseRejectionWarning: Error: Cannot determine a GraphQL host type for the "classes" field. Make sure your class is decorated with an appropriate decorator (e.g., @ObjectType()).