How to receive data from nested collection in Firebase using StreamBuilder and ListView?

I want to receive all tracks from next path:

"main_page_items" Collection -> album_id Document -> "tracks" Collection -> track_id Document

    import 'package:flutter/material.dart';
    import 'package:cloud_firestore/cloud_firestore.dart';
    import 'package:app/album.dart';
    
    class SongList extends StatelessWidget {
      final callback;
      final album;
    
      SongList(this.callback, this.album);
    
      @override
      Widget build(BuildContext context) {
        return Container(
          child: StreamBuilder<QuerySnapshot>(
            stream : FirebaseFirestore.instance
                    .collection("tracks")
                    .where("main_page_items", isEqualTo: album).snapshots(),
            builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
              final List<DocumentSnapshot> tracks = snapshot.data!.docs ;
              //print(tracks);
              print(tracks.length);
              return ListView.builder(
                itemCount: tracks.length,
                shrinkWrap: true,
                itemBuilder: (context, index) {
                  return Padding(
                    padding: const EdgeInsets.fromLTRB(0, 0, 16, 12),
                    child: SongItem(
                        tracks[index]['title'],
                        tracks[index]['artist'],
                        tracks[index]['image'],
                        tracks[index]['url'],
                        callback),
                  );
                },
              );
              
            },
          ),
        );
      }
    }
    
    class SongItem extends StatelessWidget {
  final title;
  final artist;
  final image;
  final url;
  final callback;
  SongItem(this.title, this.artist, this.image, this.url, this.callback);
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        GestureDetector(
          onTap: () {
            callback(title, artist, image, url);
          },
          child: Row(
            children: <Widget>[
              Stack(
                children: <Widget>[
                  Container(
                    height: 60.0,
                    width: 60.0,
                    child: ClipRRect(
                      borderRadius: BorderRadius.circular(8.0),
                      child: Image.network(
                        image,
                        fit: BoxFit.cover,
                      ),
                    ),
                  ),
                ],
              ),
              SizedBox(width: 16.0),
              Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text(
                    title,
                    style: TextStyle(
                        color: Colors.white,
                        fontWeight: FontWeight.bold,
                        fontSize: 18.0),
                  ),
                  SizedBox(height: 8.0),
                  Text(
                    artist,
                    style: TextStyle(
                        color: Colors.white.withOpacity(0.5), fontSize: 14.0),
                  ),
                ],
              ),
            ],
          ),
        ),
      ],
    );
  }
}

But if i've got a "0" value of length of snapshot, and if i try to print the docs (tracks variable), i've got an empty [], but i have one element in the Firestore DB.
What is the problem and how to fix it? Thank you.

id doesn't seem that the database links that you said it wat actually you used in the code :

 FirebaseFirestore.instance
.collection("tracks")
.where("main_page_items", isEqualTo: album).snapshots(),

by using this, you're likely refer to tracks as a the most parent collection in your database, but from what you provided, that tracks is a sub-collection and should be accessed like this:

FirebaseFirestore.instance
.collection("main_page_items")
.document("album_id")
.collection("tracks")
.snapshots(),