Nest.js: Circular dependencies in dynamic modules

Question

What is the correct way to import dynamic modules when there's a circular dependency between them? I simply changed forwardRef(() => MyModule) to forwardRef(() => MyModule.forRoot()), and I'm getting Nest can't resolve dependencies error.

Answer 1

The best way to deal with nest module dependency is to import dependencies always through modules and never through services. If you run into a circular dependency, nest will show you an error and you can easily fix it with forwardRef().

let say your application has 3 tightly coupled modules.

• @moduleA()
• @moduleB()
• @moduleC()

And two supporting modules

• @moduleX()
• @moduleY()
• @moduleZ()

Also, all modules are exporting services with the same name.

Consider a situation where

@moduleA() imports [serviceB, serviceX]

And

@moduleB() imports [serviceA, serviceY, serviceZ]

Now in @moduleC() if you want to use serivceA() then you will have to

@moduleC() imports [serviceA, serviceB, serviceX, serviceY, serviceZ]

In most cases, nest throws the correct error saying which dependency is missing where. But some times, nest only says dependency at [index] is missing, nest doesn't says where it is missing. This will lead to great level of confusion.

A neat approach is to always import modules

@moduleA() imports [moduleB, moduleX]

@moduleB() imports [moduleA, moduleY, moduleZ]

@moduleC() imports [moduleA]

---

If nest is complaining again about dependency issue then import modules using forwardRef

@module({
     imports: [
       forwardRef(() => ModuleA)
       ModuleB
       ],
    controllers: [],
    providers: []
})

---

Sometimes even after doing all these, you may again run into dependency not available error. Then you can use ModuleRef. Let's take the same example where you want to use ServiceA() in ModuleC()

You will then be adding serviceA() to providers[] in ModuleC() and in the serviceC() class you need to add the following.

import { Injectable, OnModuleInit } from '@nestjs/common'; \\need OnModuleInit 
import { ServiceA} from '../FolderA/serviceA.service';

export class ServiceC implements OnModuleInit {
   
   private serviceA: ServiceA;
   constructor(private readonly moduleRef: ModuleRef) {}

   onModuleInit() {
        this.serviceA= this.moduleRef.get(ServiceA);
    }
   
   foo(){
     console.log(this.serviceA.someFunction())
   }
}

Please check more at Official documentation.