如何解决“返回类型'DocumentReference（）'不是'DocumentReference（）'，由方法”错误定义？

Question

I have 3 functions, doesNameAlreadyExist checks whether a document is exists or not. I'm open to any improvement on those methods by the way.

   Future<bool> doesNameAlreadyExist(String name) async {

      QuerySnapshot queryDb = await Firestore.instance
          .collection('locations')
          .where("city", isEqualTo: '${name}')
          .limit(1)
          .getDocuments();
      final List<DocumentSnapshot> documents = queryDb.documents;
      return documents.length == 1;
// I have to return DocumentReference if document is exists,
// Even though, it's not on the scope of this particular problem,
// I'm open to ideas. Maybe I can return a map, bool and reference combined


    }

This one pushes document on firestore.

    Future<DocumentReference> pushNameToFirestore(PlaceDetails pd) async {

      Future<DocumentReference> justAddedRef = Firestore.instance.collection('locations').add(<String, String>{
        'city': '${pd.name}',
        'image': '${buildPhotoURL(pd.photos[0].photoReference)}',
      });
      return justAddedRef;
    }

And here, I'm checking and then pushing with those functions above. However I can't return the document reference.

    DocumentReference firestoreCheckAndPush() async {
     bool nameExists =  await doesNameAlreadyExist(placeDetail.name);
     // TODO notify user with snackbar and return reference
     DocumentReference locationDocumentRef;
     if(nameExists) {
       print('name exist');
     } else {
         locationDocumentRef = await pushNameToFirestore(placeDetail);
     }
        return locationDocumentRef; // Error is here
    }

Answer 1

I think the problem here is that you don't wait for your DocumentReference , so you return a Future instead of a DocumentReference which is actually expected.

Try this:

    Future<DocumentReference> pushNameToFirestore(PlaceDetails pd) async {

      DocumentReference justAddedRef = await Firestore.instance.collection('locations').add(<String, String>{
        'city': '${pd.name}',
        'image': '${buildPhotoURL(pd.photos[0].photoReference)}',
      });
      return justAddedRef;
    }

Here's a great article about future and async : Link to the article

Hope it's help !!