Retrieve stream value then call another stream based on the previous value using Flutter with firestore

**This posts collection and following collection are child nodes of user collection.Using below code snippets I can retrieve users posts directly. But I want to retrieve only followed users posts. If particular user followed another user. That followed users id saved under followings collection **

Container(
        height: 225.0,
        child: StreamBuilder<QuerySnapshot>(
          stream: FirebaseFirestore.instance
              .collection("users")
              .doc(userProfileID)
              .collection("posts")
              .orderBy('Post Date', descending: true)
              .snapshots(),
          builder: (context, snapshot) {
            return !snapshot.hasData
                ? ShimmerPlaceHolder().homePosts(true)
                : snapshot.data.docs.length.toString() == "0"
                    ? Container(
                        height: 100.0,
                        width: 200.0,
                        child: Column(
                          children: [
                            SizedBox(
                              height: 30.0,
                            ),
                            Text(
                              "You have no posts yet",
                              style: TextStyle(
                                  fontSize: AppTheme.AppLightTheme.fontLarge,
                                  fontFamily: 'AirbnbCereal'),
                            ),
                            Image.asset(
                              'assets/images/home_picture.png',
                              height: 100.0,
                              width: 100.0,
                            ),
                          ],
                        ),
                      )
                    : Container(
                        margin: EdgeInsets.symmetric(vertical: 20.0),
                        height: 200,
                        child: ListView.builder(
                          scrollDirection: Axis.horizontal,
                          itemCount: snapshot.data.docs.length,
                          itemBuilder: (BuildContext context, int index) {
                            Post postModel = Post.fromDocument(
                                snapshot.data.docs[index],
                                snapshot.data.docs.length);
                            return Container(
                              width: MediaQuery.of(context).size.width * 0.35,
                              child: Card(
                                //TODO: Add all images radius
                                color: AppTheme.AppLightTheme.unSelectColor,
                                child: snapshot.data.docs.length == null
                                    ? ShimmerPlaceHolder().homePosts(true)