title: TypeOrm/PostgreSQL createQueryBuilder: select column from joined table
body: <p>I want to select only a column from a table that I'm joining into. A <code>separator</code> has a <code>facility</code>, a <code>facility</code> has a <code>customer</code>, and I want to select <code>customer.id</code> by providing a <code>separatorId</code>.</p>
  <p>Below is what I have tried, but this returns a <code>Separator</code>.</p>
  <pre><code>return await this.separatorRepository
    .createQueryBuilder('separator')
    .select(['customer.companyId'])
    .innerJoin('separator.facility', 'facility')
    .innerJoin('facility.customer', 'customer')
    .where('separator.id = :separatorId', { separatorId })
    .getOne();
  </code></pre>
  <p>How do I change this query to get <code>customer.id</code>?</p>