How to override provider for tests in NestJs and not create transitive dependencies that are not needed?

Question

I am writing end-to-end tests in NestJs and using the "overrideProvider" function to setup test versions of various services (e.g. services that require database connections). I noticed though that even when I do this, the original implementation that injects the real database is still instantiated.

Is there a way to tell Nest to not create transitive dependencies that are overridden?

For example, I have a test that starts like:

...

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [ServiceModule],
    })
    // Works if I uncomment these lines:
    // .overrideProvider('Database')
    // .useValue(new TestDatabase())
    .overrideProvider('ServiceUsingDatabase')
    .useValue(new TestService())
    .compile();

...

Where the module setup is like:

import { Inject, Injectable, Module } from '@nestjs/common';

interface Database {} 

@Injectable()
class ProductionDatabase implements Database {
  constructor() {
    throw('Cannot create a production database.');
  }
}

@Injectable()
export class TestDatabase implements Database {
  constructor() {
    console.log('Creating the test database.');
  }
}

@Module({
  providers: [
    {
      provide: 'Database',
      useClass: ProductionDatabase

    }
  ],
  exports: ['Database']
})
class DatabaseModule {}


interface Service {}

@Injectable()
class ProductionService implements Service {
  constructor(@Inject('Database') private readonly database: Database) {}
}

@Injectable()
export class TestService implements Service {
  // Test implementation of the service does not Inject anything.
}

@Module({
  imports: [DatabaseModule],
  providers: [
    {
      provide: 'ServiceUsingDatabase',
      useClass: ProductionService
    }
  ],
})
export class ServiceModule {}

But, the DI system is still seeming to try and instantiate ProductionDatabase. If I explicitly override the provider for the 'Database' it works, but I'd like to avoid having to explicitly list all transitive dependencies as such.

This was a good hint thanks! The DatabaseModule had useValue(). I changed that to useFactory() and got the behavior I was hoping for. — okhobb, May 19 '22 at 02:17
This got me over one part of my question, but what I was really hoping to do was slightly different / more complicated. I'll update the question. — okhobb, May 19 '22 at 03:55
Ah, I see, no, there's no way to have the `'Database'` dependency automatically not resolved if you override the `ServiceUsingDatabase`provider. I'll need to double check, but I believe Nest creates instances of each declared dependency as soon as it reads it and is able to — Jay McDoniel, May 19 '22 at 04:21

score 1 · Accepted Answer · answered Jun 27 '22 at 05:42

I ended up deciding to make a "Test" Module for every Module e.g.:

import { Inject, Injectable, Module } from '@nestjs/common';

interface Database {} 

@Injectable()
class ProductionDatabase implements Database {
}

@Injectable()
export class TestDatabase implements Database {
}

@Module({
  providers: [
    {
      provide: 'Database',
      useClass: ProductionDatabase

    }
  ],
  exports: ['Database']
})
class DatabaseModule {}

@Module({
  providers: [
    {
      provide: 'Database',
      useClass: TestDatabase

    }
  ],
  exports: ['Database']
})
class TestDatabaseModule {}


interface Service {}

@Injectable()
class ProductionService implements Service {
  constructor(@Inject('Database') private readonly database: Database) {}
}

@Injectable()
export class TestService implements Service {
}

@Module({
  imports: [DatabaseModule],
  providers: [
    {
      provide: 'ServiceUsingDatabase',
      useClass: ProductionService
    }
  ],
})
export class ServiceModule {}


@Module({
  providers: [
    {
      provide: 'ServiceUsingDatabase',
      useClass: TestService
    }
  ],
})
export class TestServiceModule {}

etc... Though it turned out after some refactorings that the "Test" module wasn't needed as some Modules became pure business logic.

How to override provider for tests in NestJs and not create transitive dependencies that are not needed?

1 Answers1