title: Why I am getting cannot determine GraphQL output error?
body: <p>I am trying to create simple appliaction with <strong>Nest.js</strong>, <strong>GraphQL</strong> and <strong>MongoDB</strong>. I wnated to use <strong>TypeORM</strong> and <strong>TypeGraphql</strong> to generate my schema and make a connection with localhost databasebut but i can not run my server with <code>nest start</code> becouse I am getting this error:</p>
  
  <blockquote>
    <p>UnhandledPromiseRejectionWarning: Error: Cannot determine GraphQL output type for getArticles</p>
  </blockquote>
  
  <p>I have no idea why i am getting this error. My class <code>ArticleEntity</code> does't has any not primary types, so there should not be any problem. I tried to remove <code>() =&gt; ID</code> from <code>@Field()</code> decorator of filed <code>_id</code> of <code>ArticleEntity</code> class but it didn't helped</p>
  
  <p><strong>ArticleResolver</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>@Resolver(() =&gt; ArticleEntity)
  export class ArticlesResolver {
    constructor(
      private readonly articlesService: ArticlesService) {}
  
    @Query(() =&gt; String)
    async hello(): Promise&lt;string&gt; {
      return 'Hello world';
    }
  
    @Query(() =&gt; [ArticleEntity])
    async getArticles(): Promise&lt;ArticleEntity[]&gt; {
      return await this.articlesService.findAll();
    }
  
  }
  </code></pre>
  
  <p><strong>ArticleService</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>@Injectable()
  export class ArticlesService {
    constructor(
      @InjectRepository(ArticleEntity)
      private readonly articleRepository: MongoRepository&lt;ArticleEntity&gt;,
    ) {}
  
    async findAll(): Promise&lt;ArticleEntity[]&gt; {
      return await this.articleRepository.find();
    }
  }
  </code></pre>
  
  <p><strong>ArticleEntity</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>@Entity()
  export class ArticleEntity {
    @Field(() =&gt; ID)
    @ObjectIdColumn()
    _id: string;
  
    @Field()
    @Column()
    title: string;
  
    @Field()
    @Column()
    description: string;
  }
  </code></pre>
  
  <p><strong>ArticleDTO</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>@InputType()
  export class CreateArticleDTO {
    @Field()
    readonly title: string;
  
    @Field()
    readonly description: string;
  }
  </code></pre>
  
  <p>If you need anything else comment</p>