How to fetch a `DocumentReference` from a Firebase `get()` - flutter

Question

I have a collection ads that contains a DocumentReference as ownerId.
With the code below, I am able to fetch the 10 most recent ads as aList<Ad>:
/// Returns a list of ads of the given [category]
static Future<List<ClassifiedAd>> getFromCategory(
ClassifiedAdCategory category,
{int max = 10}) async {
return FirebaseFirestore.instance
.collection('ads')
.where('category', isEqualTo: category.name)
.orderBy('creationDate', descending: true)
.limit(max)
.get()
.then((snapshot) {
return snapshot.docs.map((doc) {
final data = doc.data();
return Ad.fromMap(data);
}).toList();
});
But now I'd like to fetch the owner (collection users) from the DocumentReference I was talking about above. But I am a but puzzled about how to do that.
My modified code below does not compile:
The return type 'List' isn't a 'FutureOr<List>', as required by the closure's context.
/// Returns a list of ads of the given [category]
static Future<List<ClassifiedAd>> getFromCategory(
ClassifiedAdCategory category,
{int max = 10}) async {
return FirebaseFirestore.instance
.collection('ads')
.where('category', isEqualTo: category.name)
.orderBy('creationDate', descending: true)
.limit(max)
.get()
.then((snapshot) {
// <<<< Error starts right here down to the removeWhere()
return snapshot.docs.map((doc) {
final data = doc.data();
final DocumentReference docRef = data["ownerId"];
return docRef.get().<ClassifiedAd?>then((snapshot) {
if (snapshot.exists) {
return ClassifiedAd.fromMap(data);
}
return null;
});
}).toList()
// Don't take into account potential nulls
..removeWhere((a) => a == null);
});
How should I do that?

Answer 1

I would say that the wrong thing that you're doing is you're trying to get a snapshot asynchronously inside the map() method which is synchronous, for such cases like yours, I recommend using await/async and to not return anything until you guarantee that you got it, try this:
static Future<List<ClassifiedAd>> getFromCategory(
ClassifiedAdCategory category,
{int max = 10}) async {
final snapshot = await FirebaseFirestore.instance
.collection('ads')
.where('category', isEqualTo: category.name)
.orderBy('creationDate', descending: true)
.limit(max)
.get();
List<ClassifiedAd> result = [];
for (int index = 0; index < snapshot.docs.length; index++) {
final doc = snapshot.docs[index];
final data = doc.data();
final DocumentReference docRef = data["ownerId"];
final docOwnerSnapshot = await docRef.get();
if (docOwnerSnapshot.exists) {
result.add(ClassifiedAd.fromMap(data));
}
}
return result;
}