typeorm - What is the best practice for avoiding (or handling) circular dependency in NestJS when needed for table relationships

admin管理员组

文章数量:1287828

I'm working on building an API for a small hackathon project. It will be a platform for teachers to share lesson plans for students.

I created my first table, Subject, to store lesson subjects like math, science, etc. and I'm now creating a second table SubSubject (I'm not crazy about the name either) for sub-genres (ex. Science would include biology, chemistry, etc.).

I'm using typeorm for generating migrations and have already created and run the first migration to create the Subject table. I then created the entity for SubSubject and modified the Subject entity to include the relationship between the two tables.

My problem now is that I've updated the modules to include the entities and forwardRefs to try to resolve the circular dependency, but it doesn't solve that error about one entity (in my case, the SubjectEntity) not being accessed before initialization. I assumed from reading the documents that a fix would be as simple as adding a forwardRef to each entity's module, but that doesn't seem to be the case.

I'm wondering if there's something fundamental that I'm not understanding about how the forwardRef works, so any explanation to help with future debugging is appreciated. I also have read several other SO posts about resolving circular dependencies, and many of them seem to imply that if you're needing to fix a circular dependency, there's a code smell there. Should I not be establishing the ManyToOne and OneToMany on each side? Is there a simpler way that I should be establishing these relations between tables? Some of the articles I read were more related to service dependencies being circular, but I'm just trying to establish table relationships here.

Here are entity and module files so far, since they're pretty small:

Subject Module:

import { forwardRef, Module } from '@nestjs/common';
import { SubjectEntity } from './subject.entity.js';
import { SubjectController } from './subject.controller.js';
import { SubjectService } from './subject.service.js';
import { SubSubjectModule } from '../sub-subject/sub-subject.module.js';
import { TypeOrmModule } from '@nestjs/typeorm';

@Module({
  imports: [
    TypeOrmModule.forFeature([SubjectEntity]),
    forwardRef(() => SubSubjectModule),
  ],
  controllers: [SubjectController],
  providers: [SubjectService],
})
export class SubjectModule {}

Subject Entity:

import { BaseEntity } from '../../utils/base.entity.js';
import { SubSubjectEntity } from '../sub-subject/sub-subject.entity.js';

@Entity('subject')
@Unique(['name'])
export class SubjectEntity extends BaseEntity {
  @Column({ type: 'varchar', length: 100 })
  name: string;

  @OneToMany(() => SubSubjectEntity, (subSubject) => subSubject.subject)
  subSubjects: SubSubjectEntity[];
}

SubSubjectModule:

import { forwardRef, Module } from '@nestjs/common';
import { SubSubjectService } from './sub-subject.service.js';
import { SubSubjectEntity } from './sub-subject.entity.js';
import { SubjectModule } from '../subject/subject.module.js';
import { TypeOrmModule } from '@nestjs/typeorm';

@Module({
  imports: [
    TypeOrmModule.forFeature([SubSubjectEntity]),
    forwardRef(() => SubjectModule),
  ],
  providers: [SubSubjectService],
  exports: [SubSubjectService],
})
export class SubSubjectModule {}

SubSubjectEntity:

import { Entity, Column, Unique, ManyToOne, JoinColumn } from 'typeorm';
import { BaseEntity } from '../../utils/base.entity.js';
import { SubjectEntity } from '../subject/subject.entity.js';

@Entity('sub_subject')
@Unique(['name'])
export class SubSubjectEntity extends BaseEntity {
  @Column({ type: 'varchar', nullable: false, length: 100 })
  name: string;

  @ManyToOne(() => SubjectEntity, (subject) => subject.subSubjects, {
    onDelete: 'RESTRICT',
    nullable: false,
  })
  @JoinColumn({ name: 'subject_id' })
  subject: SubjectEntity;
}

I have tried removing the entity import from each and that temporarily allowed the API to run locally, but then I was unable to run my migration command and received the same error:

ReferenceError: Cannot access 'SubjectEntity' before initialization

I also considered that maybe the forwardRef is only needed in one file or the other, and that having it in both places was repeating the circular dependency I was trying to fix, but I tried removing it from each module and that did not resolve the issue either.

I'm working on building an API for a small hackathon project. It will be a platform for teachers to share lesson plans for students.

I created my first table, Subject, to store lesson subjects like math, science, etc. and I'm now creating a second table SubSubject (I'm not crazy about the name either) for sub-genres (ex. Science would include biology, chemistry, etc.).

I'm using typeorm for generating migrations and have already created and run the first migration to create the Subject table. I then created the entity for SubSubject and modified the Subject entity to include the relationship between the two tables.

My problem now is that I've updated the modules to include the entities and forwardRefs to try to resolve the circular dependency, but it doesn't solve that error about one entity (in my case, the SubjectEntity) not being accessed before initialization. I assumed from reading the documents that a fix would be as simple as adding a forwardRef to each entity's module, but that doesn't seem to be the case.

I'm wondering if there's something fundamental that I'm not understanding about how the forwardRef works, so any explanation to help with future debugging is appreciated. I also have read several other SO posts about resolving circular dependencies, and many of them seem to imply that if you're needing to fix a circular dependency, there's a code smell there. Should I not be establishing the ManyToOne and OneToMany on each side? Is there a simpler way that I should be establishing these relations between tables? Some of the articles I read were more related to service dependencies being circular, but I'm just trying to establish table relationships here.

Here are entity and module files so far, since they're pretty small:

Subject Module:

import { forwardRef, Module } from '@nestjs/common';
import { SubjectEntity } from './subject.entity.js';
import { SubjectController } from './subject.controller.js';
import { SubjectService } from './subject.service.js';
import { SubSubjectModule } from '../sub-subject/sub-subject.module.js';
import { TypeOrmModule } from '@nestjs/typeorm';

@Module({
  imports: [
    TypeOrmModule.forFeature([SubjectEntity]),
    forwardRef(() => SubSubjectModule),
  ],
  controllers: [SubjectController],
  providers: [SubjectService],
})
export class SubjectModule {}

Subject Entity:

import { BaseEntity } from '../../utils/base.entity.js';
import { SubSubjectEntity } from '../sub-subject/sub-subject.entity.js';

@Entity('subject')
@Unique(['name'])
export class SubjectEntity extends BaseEntity {
  @Column({ type: 'varchar', length: 100 })
  name: string;

  @OneToMany(() => SubSubjectEntity, (subSubject) => subSubject.subject)
  subSubjects: SubSubjectEntity[];
}

SubSubjectModule:

import { forwardRef, Module } from '@nestjs/common';
import { SubSubjectService } from './sub-subject.service.js';
import { SubSubjectEntity } from './sub-subject.entity.js';
import { SubjectModule } from '../subject/subject.module.js';
import { TypeOrmModule } from '@nestjs/typeorm';

@Module({
  imports: [
    TypeOrmModule.forFeature([SubSubjectEntity]),
    forwardRef(() => SubjectModule),
  ],
  providers: [SubSubjectService],
  exports: [SubSubjectService],
})
export class SubSubjectModule {}

SubSubjectEntity:

import { Entity, Column, Unique, ManyToOne, JoinColumn } from 'typeorm';
import { BaseEntity } from '../../utils/base.entity.js';
import { SubjectEntity } from '../subject/subject.entity.js';

@Entity('sub_subject')
@Unique(['name'])
export class SubSubjectEntity extends BaseEntity {
  @Column({ type: 'varchar', nullable: false, length: 100 })
  name: string;

  @ManyToOne(() => SubjectEntity, (subject) => subject.subSubjects, {
    onDelete: 'RESTRICT',
    nullable: false,
  })
  @JoinColumn({ name: 'subject_id' })
  subject: SubjectEntity;
}

I have tried removing the entity import from each and that temporarily allowed the API to run locally, but then I was unable to run my migration command and received the same error:

ReferenceError: Cannot access 'SubjectEntity' before initialization

I also considered that maybe the forwardRef is only needed in one file or the other, and that having it in both places was repeating the circular dependency I was trying to fix, but I tried removing it from each module and that did not resolve the issue either.

• nestjs
• typeorm
• circular-dependency

Share Improve this question Follow edited Feb 22 at 23:26 Paul T. 4,9171212 gold badges4747 silver badges6363 bronze badges asked Feb 22 at 20:53 AllisonAllison 3111 silver badge77 bronze badges

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 0

Ensure that your services are not causing circular dependencies. If your SubjectService depends on SubSubjectService and vice versa, you might need to use forwardRef in the service providers as well. Have you tried this?

@Injectable()
export class SubjectService {
  constructor(
    @Inject(forwardRef(() => SubSubjectService))
    private readonly subSubjectService: SubSubjectService,
  ) {}
}

本文标签：