title: How to query a document collection
body: <p>I'm newbie at AngularFire and I'm trying to use Firestore Database with this structure:</p>
  <pre class="lang-sh prettyprint-override"><code>/departments/&lt;department_document_id&gt;/workers/&lt;worker_document_id&gt;
  </code></pre>
  <p>Each <em>Department</em> document has some fields and a collection of workers, and each <em>Worker</em> document has it own fields, obviously.</p>
  <p>I'm trying to get the workers list of a specific department, so in my DataService:</p>
  <pre class="lang-js prettyprint-override"><code>getWorkers(departmentId: string) {
      this.departmentsCollection = this.afs.collection('departments', ref =&gt; ref.where('departmentId', '==', departmentId));
      // (?)
  }
  </code></pre>
  <p>I supose that I should use <em>snapshotChanges()</em> with a <em>pipe()</em> in order to get the <em>DocumentChangeType</em> and query the <em>workers</em> collection, but I'm stucked at this point. I was trying it in different ways without results.</p>
  <p>Any help / suggestions?</p>
  <p>Edit: note that <em>departmentId</em> param is a field of a Department document, not the Firestore autogenerated ID.</p>