Undefined class 'AllCoursesSnapshot' - firebase

Question

I am trying to retrieve data from firebase database and display it on my UI. This is the code written :
class AllCourses {
String courseName;
String teacher;
String category;
AllCourses(this.courseName, this.teacher, this.category);
}
import 'package:creators_club/firestore/courses.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
class BusinessPage extends StatefulWidget {
#override
_BusinessPageState createState() => _BusinessPageState();
}
class _BusinessPageState extends State<BusinessPage> {
List<AllCourses> coursesList = [];
#override
void initState(){
super.initState();
DatabaseReference referenceAllCourses = FirebaseDatabase.instance.reference().child('AllCourses');
referenceAllCourses.once().then(((AllCoursesSnapshot allCoursesSnapshot){
coursesList.clear();
var keys = allCoursesSnapshot.value.keys;
var values = allCoursesSnapshot.value;
for(var key in keys){
AllCourses allCourses = new AllCourses(
values [key]["courseName"],
values [key]["teacher"],
values [key]["category"],
);
coursesList.add(allCourses);
}
setState(() {
//HERE
});
}));
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
leading: IconButton(
icon: Icon(Icons.arrow_back, color: Colors.white),),
title: Text("Creator's Club"),
backgroundColor: Color(0xff2657ce),
elevation: 0,),
body: coursesList.length == 0 ? Center(child: Text("No Data Avail", style: TextStyle(fontSize: 15),)): ListView.builder(
itemCount: coursesList.length,
itemBuilder: (_,index){
return CardUI(coursesList[index].courseName, coursesList[index].teacher, coursesList[index].category);}
)
);
}
}
}
Why isn't the AllCoursesSnapshot being recognized? It says that it is "Type: Null Function(dynamic)".
Here is a picture of the database table in Realtime Database: (https://imgur.com/a/xDZy1SW).

Answer 1

It is not recognized because there is no class called AllCoursesSnapshot. The once() method belongs to the firebase_database plugin, and it returns a Future<DataSnapshot>, therefore you have to do the following:
referenceAllCourses.once().then((DataSnapshot dataSnapshot){
Also replace allCoursesSnapshot with dataSnapshot in the other parts of your code.