title: Get a list of firestore documents from array of id's
body: <p>I've get a firestore collection,  one of his values is an array of documents id.
  I need to retrieve all items of collection and also all documents stored in array ids.
  this is the struct of my collection
  <a href="https://i.stack.imgur.com/rA8pj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rA8pj.png" alt="enter image description here"></a></p>
  
  <p><strong>my code:</strong></p>
  
  <pre><code>export const getFeaturedMixes = functions.https.onRequest((request, response) =&gt; {
      let result:any[] = []
      featuredMixes.get().then(mixesSnap =&gt; {
  
          mixesSnap.forEach(doc =&gt; {
              let docId = doc.id
              let ids = doc.data().tracks
  
              let resultTracks:any[] = []
              ids.forEach(id =&gt; {
                  let t = tracksCollection.doc(id.track_id).get()
                  resultTracks.push({'id' : id.track_id, 'data': t})
              })
              result.push({'id':docId, 'tracks': resultTracks})
          })
          return result
      })
      .then(results =&gt; {
          response.status(200).send(results)
      }).catch(function(error) {
          response.status(400).send(error)
      })
  });
  </code></pre>
  
  <p><strong>and I get this response:</strong></p>
  
  <pre><code>{
          "id": "Xm4TJAnKcXJAuaZr",
          "tracks": [
              {
                  "id": "FG3xXfldeJBbl8PY6",
                  "data": {
                      "domain": {
                          "domain": null,
                          "_events": {},
                          "_eventsCount": 1,
                          "members": []
                      }
                  }
              },
              {
                  "id": "ONRfLIh89amSdcLt",
                  "data": {
                      "domain": {
                          "domain": null,
                          "_events": {},
                          "_eventsCount": 1,
                          "members": []
                      }
                  }
              }
      ]
  }
  </code></pre>
  
  <p><strong>response doesn't contains document data</strong></p>