title: Angularfire2 & Firestore – retrieve all subcollection content for a collection list
body: <p>I try to retrieve datas in a <code>subcollection</code> based on the key received on the first call.
  Basically, I want a list of all my user with the total of one subcollection for each of them.</p>
  
  <p>I'm able to retrieve the data from the first Payload, but not from <code>pointRef</code> below</p>
  
  <p>What is the correct way to achieve that?</p>
  
  <pre><code>getCurrentLeaderboard() {
      return this.afs.collection('users').snapshotChanges().map(actions =&gt; {
        return actions.map(a =&gt; {
          const data = a.payload.doc.data()
          const id = a.payload.doc.id;
          const pointRef: Observable&lt;any&gt; = this.afs.collection('users').doc(`${id}`).collection('game').valueChanges()
  
          const points = pointRef.map(arr =&gt; {
            const sumPoint = arr.map(v =&gt; v.value)
            return sumPoint.length ? sumPoint.reduce((total, val) =&gt; total + val) : ''
          })
  
          return { id, first_name: data.first_name, point:points };
        })
      })
    }
  </code></pre>