title: Error : Nest cannot export component / module that is not a part of the currently processed module (DatabaseModule)
body: <p>I'm programming a really simple app allowed make CRUD feature on a PostgreSQL DB using express.
  When I run my program I get this error : 
  Error: Nest cannot export component / module that is not a part of the currently proccessed module (DatabaseModule). Please verify whether each exported unit is available in this particular context.</p>
  
  <p>I really don't understand why... In the app.module.ts, I import DatabaseModule, which calls database provider (where is my postreSQL connection).</p>
  
  <p>I'm new on typescript, and I'm lost : / 
  I only post the entry point of app, but I can send more if it's not enough and thank's for the help : </p>
  
  <pre><code>import { NestFactory } from '@nestjs/core';
  import { ApplicationModule } from './src/app.module';
  import * as express from 'express';
  import 'ts-node/register';
  
  async function bootstrap() {
      let instance: any;
      instance = express();
      const app = NestFactory.create(ApplicationModule, instance);
      app.then(instance =&gt; instance.listen(3000), () =&gt; console.log('Application is listening on port 3000'));
  }
  
  bootstrap();
  </code></pre>
  
  <p>This app.module.ts</p>
  
  <pre><code>import { Module } from '@nestjs/common';
  import { MainController } from './controller/main.controller';
  
  import { CatsModule } from './cats.module';
  import { CatsController } from './controller/cats.controller';
  
  import { OwnerModule } from './owner.module';
  import { OwnerController } from './controller/owner.controller';
  
  import {CatfoodController} from './controller/catfood.controller';
  import { CatfoodModule } from './catfood.module';
  
  import { DatabaseModule } from './database/database.module';
  
  @Module({
    controllers: [
        MainController,
        CatsController,
        CatfoodController,
        OwnerController,
    ],
      modules: [
          DatabaseModule,
          CatsModule,
          CatfoodModule,
          OwnerModule,
      ],
  })
  
  export class ApplicationModule {}
  </code></pre>
  
  <p>And database.module :</p>
  
  <pre><code>import { Module } from '@nestjs/common';
  import { databaseProviders } from './database.providers';
  
  @Module({
      components: [...databaseProviders],
      exports: [...databaseProviders],
  })
  export class DatabaseModule {}
  </code></pre>
  
  <p>Github : <a href="https://github.com/lukile/catProject" rel="noreferrer">https://github.com/lukile/catProject</a></p>