Flutter : How to get the length of field (list) in Firestore document?

You will probably need to use a nested ListView.builder, because you want to get the data of a particular user first, which you can do in a parent ListView. Once you get the current user data, you can use its "groups" property in a child ListView.

Here’s an example:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance.collection("users").snapshots(),
        builder: (context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasData) {
            final isCollectionEmpty = snapshot.data!.docs.isEmpty;
            final DocumentsWhichContainsPosts = snapshot.data!.docs.where(
                (doc) =>
                    (doc.data() as Map<String, dynamic>)["groups"].isNotEmpty);
            
            final mappedData =
              DocumentsWhichContainsPosts.map((DocumentSnapshot doc) =>
                                              (doc.data() as Map<String, dynamic>)).toList(); // Map the data in docs into a List of Map<String, dynamic>
            
            if (DocumentsWhichContainsPosts.isNotEmpty) {
              
              return ListView.builder( // Users List View
                itemCount: mappedData.length, // here, use the length of all the docs you have in a collection
                itemBuilder: ((context, index) {
                  final currentData = mappedData[index];
                  final groups = currentData['groups'];
                  return ListView.builder( // Groups List View
                    itemCount: groups.length,
                    itemBuilder: (context, i) {
                      return Text(groups[index]); // Now you can loop through the groups
                    }
                  );
                }),
              );
            } else {
              return Container(
                child: Center(child: Text("No posts")),
              );
            }
          } else {
            return const Center(
              child: CircularProgressIndicator(color: Colors.red),
            );
          }
        });;
  }
}