Overriding providers in NestJS Jest tests

Question

I want to use an in-memory Mongo instance to mock data for testing purposes in my NestJS application. I have a database provider which connects to my production db using mongoose, which is part of my database module, which in turn gets imported into other modules.

I am trying to override the database provider within my Jest tests so I can use the in-memory Mongo instance.

This is the database module:

import { Module } from '@nestjs/common';
import { databaseProviders } from './database.providers';

@Module({
  providers: [...databaseProviders],
  exports: [...databaseProviders],
})
export class DatabaseModule { }

and the databaseProvider:

export const databaseProviders = [
  {
    provide: 'DbConnectionToken',
    useFactory: async (): Promise<typeof mongoose> =>
      await mongoose.connect(PRODUCTION_DATABASE_URL),
  },
];

I have an Events module which imports and uses the database connection from the database module the Events service is what I am testing - the beforeEach in my events.spec.ts:

beforeEach(async () => {
    const module = await Test.createTestingModule({
      imports: [EventsModule],
      providers: [
        EventsService,
        {
          provide: 'EventModelToken',
          useValue: EventSchema
        },
      ],
    }).compile();

    eventService = module.get<EventsService>(EventsService);
  });

I tried importing the DatabaseModule into the testing module and then adding my custom provider assuming it would override the database provider, but it doesn't work as I expected so I fear I may misunderstand how overriding providers works in this context.

This is what I tried:

beforeEach(async () => {
  const module = await Test.createTestingModule({
    imports: [EventsModule, DatabaseModule],
    providers: [
      EventsService,
      {
        provide: 'EventModelToken',
        useValue: EventSchema
      },
      {
        provide: 'DbConnectionToken',
        useFactory: async (): Promise<typeof mongoose> =>
          await mongoose.connect(IN_MEMORY_DB_URI),
      },
    ],
  }).compile();

  eventService = module.get<EventsService>(EventsService);
});

Answer 1

As specified in the docs https://docs.nestjs.com/fundamentals/unit-testing you can override the provider with a value, factory or class.

beforeEach(async () => {
    const module = await Test.createTestingModule({
        imports: [EventsModule, DatabaseModule],
        providers: [
            EventsService,
        ],
    }).overrideProvider('DbConnectionToken')
    .useFactory({
        factory: async (): Promise<typeof mongoose> =>
          await mongoose.connect(IN_MEMORY_DB_URI),
    })
    .compile();

    eventService = module.get<EventsService>(EventsService);
});

An alternative would be to make a provider for your configs :) Like so!

@Module({})
export DatabaseModule {
    public static forRoot(options: DatabaseOptions): DynamicModule {
        return {
            providers: [
                {
                    provide: 'DB_OPTIONS',
                    useValue: options,
                },
                {
                    provide: 'DbConnectionToken',
                    useFactory: async (options): Promise<typeof mongoose> => await mongoose.connect(options),
                    inject: ['DB_OPTIONS']
                },
            ],
        };
    }
}

Then use like this

const module: TestingModule = await Test.createTestingModule({
    imports: [DatabaseModule.forRoot({ host: 'whatever'})],
});

Now you're able to change the options wherever you're using it :)