title: TypeORM: Load relations with only certain specified fields
body: <p>Despite much research and trial and error, I unfortunately do not yet have an elegant solution for the following use case:
  A user has several posts, which is correctly implemented as a <strong>one-to-many relationship</strong>. I want to load the user with all his posts, but <strong>not all attributes</strong> of the posts, <strong>only certain ones</strong>.</p>
  
  <p><em>Approach 1: Find</em></p>
  
  <pre><code>await this.usersRepository.find({relations: ['posts']});
  </code></pre>
  
  <p>Here I can't define a corresponding select restriction anywhere.</p>
  
  <p><em>Approach 2: QueryBuilder</em></p>
  
  <pre><code>await this.usersRepository.createQueryBuilder('user')
  .leftJoinAndSelect('user.posts', 'posts').select('posts.title').getMany();
  </code></pre>
  
  <p>Here I define the restriction supposedly, but the select seems to refer only to user.</p>
  
  <p>Am I missing something? Or is there no intended solution for my use case?</p>
  
  <p>I would be very grateful for help, thanks guys!</p>
  
  <p>TypeORM version 0.2.22</p>