Deleting document from cloud_firestore in flutter

doc() will generate a new random id, therefore if you don't have access to the id, then you need to do the following:

    FirebaseFirestore.instance
        .collection("eventDetails")
        .where("chapterNumber", isEqualTo : "121 ")
        .get().then((value){
          value.docs.forEach((element) {
           FirebaseFirestore.instance.collection("eventDetails").doc(element.id).delete().then((value){
             print("Success!");
           });
          });
        });

Use a where() condition to get the required document and delete it.

Since in your code you are using:

return StreamBuilder<QuerySnapshot>(
      stream: firestoreInstance.collection('eventDetails').snapshots(),

Here you are fetching all the documents under eventDetails, therefore you can add a unique field to the document and then inside the for loop you can get the id:

for (var i in allDocuments) {
          if(i.data()["subject"] == "Mathemtics")
               docId = i.id;

And then you can delete it:

              onDismissed: (direction) {
                FirebaseFirestore.instance
                    .collection("eventDetails")
                    .doc(docId)
                    .delete()
                    .then((_) {
                  print("success!");
                });
              },

This way you dont need to fetch the documents twice.

You can always query the document to get the document ID and perform the deletion.

var collection = FirebaseFirestore.instance.collection('users');
var snapshot = await collection.where('age', isGreaterThan: 20).get();
for (var doc in snapshot.docs) {
 await doc.reference.delete();
}

To delete all the documents, iterate through the QueryDocumentSnapshot.

var collection = FirebaseFirestore.instance.collection('collection');
var querySnapshots = await collection.get();
for (var doc in querySnapshots.docs) {
  await doc.reference.delete();
}

I just had to use,
i.id
to get the document id. And I could delete the document from the firestore by swipping.

Here is the complete code,

 showingTheSelectedDateEvents() {
    List<Widget> listViewContainer = [];

    return StreamBuilder<QuerySnapshot>(
      stream: firestoreInstance.collection('eventDetails').snapshots(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return Center(
            child: CircularProgressIndicator(
              backgroundColor: Colors.lightBlueAccent,
            ),
          );
        }
        String theDatabaseDate;
        final allDocuments = snapshot.data.docs;
        //here we get all the documents from the snapshot.
        for (var i in allDocuments) {
          // if(i.data()["subject"] == "Mathemtics")

          theDatabaseDate = i.data()['dateTime'];
          if (theDatabaseDate == theDataProvider.databaseSelectedDate) {
            print(theDatabaseDate +
                " is same as " +
                theDataProvider.databaseSelectedDate);
            listViewContainer.add(Dismissible(
              key: ObjectKey(snapshot.data.docs.elementAt(0)),
              onDismissed: (direction) async {
                firestoreInstance.collection("eventDetails").doc(i.id).delete();

                // allDocuments.removeAt(0);
              },
              background: Container(
                alignment: Alignment.centerRight,
                child: Padding(
                  padding: const EdgeInsets.fromLTRB(0, 0, 20, 0),
                  child: Text(
                    "Slide To Remove",
                    style: TextStyle(
                        fontSize: 15,
                        fontFamily: "Poppins",
                        color: Colors.black),
                  ),
                ),
                decoration: BoxDecoration(color: Color(0xFFF4F5F6)),
              ),
              child: Widgets(
                containerHeight: 170,
                thePaddingValue:
                    EdgeInsets.only(left: 90, right: 10, top: 20, bottom: 20),
                iconSize: 60,
                chapterName: i.data()['chapterName'],
                chapterNumber: i.data()['chapterNumber'],
                meetType: i.data()['meetType'],
                meetIcon: Icons.video_call,
                subject: i.data()['subject'],
                lectureTime: "09:30",
                teacherImage:
                    AssetImage("assets/images/IMG-20200817-WA0000.jpg"),
                teacherName: "Alex Jesus",
              ),
            ));
            print(listViewContainer.length);
          } else {
            print("no any events for today");
          }
        }
        return Expanded(
          child: ListView(
            reverse: true,
            padding: EdgeInsets.symmetric(horizontal: 10.0, vertical: 20.0),
            children: listViewContainer,
          ),
        );
      },
    );
  }