Flutter Sqflite error says The method 'query' was called on null - dart

Question

I am trying to fetch some data from network and store it in sqlite database. Following is the model class
class StudentModel {
int status;
String msg;
StudentModelData studentModelData;
StudentModel({this.status, this.msg, this.studentModelData});
StudentModel.fromJson(Map<String, dynamic> json) {
status = json['status'];
msg = json['msg'];
studentModelData = json['data'] != null ? new StudentModelData.fromJson(json['data']) : null;
}
StudentModel.fromDb(Map<String, dynamic> parsedJson) {
status = parsedJson['status'];
msg = parsedJson['msg'];
studentModelData = studentModelData = jsonDecode(json['data']) != null ? new StudentModelData.fromJson(jsonDecode(json['data'])) : null;
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['status'] = this.status;
data['msg'] = this.msg;
if (this.studentModelData != null) {
data['data'] = this.studentModelData.toJson();
}
return data;
}
}
class StudentModelData {
int lastIndex;
List<StudentData> studentData;
StudentModelData({this.lastIndex, this.studentData});
StudentModelData.fromJson(Map<String, dynamic> json) {
lastIndex = json['lastIndex'];
if (json['studentData'] != null) {
studentData = new List<StudentData>();
json['studentData'].forEach((v) {
studentData.add(new StudentData.fromJson(v));
});
}
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['lastIndex'] = this.lastIndex;
if (this.studentData != null) {
data['studentData'] = this.studentData.map((v) => v.toJson()).toList();
}
return data;
}
}
class StudentData {
String studentId;
String studName;
String studProfilepic;
String studentEmail;
String studentMobile;
String courseName;
String classCode;
int minAvg;
int avg;
StudentData(
{this.studentId,
this.studName,
this.studProfilepic,
this.studentEmail,
this.studentMobile,
this.courseName,
this.classCode,
this.minAvg,
this.avg});
StudentData.fromJson(Map<String, dynamic> json) {
studentId = json['student_id'];
studName = json['stud_name'];
studProfilepic = json['stud_profilepic'];
studentEmail = json['student_email'];
studentMobile = json['student_mobile'];
courseName = json['course_name'];
classCode = json['class_code'];
minAvg = json['minAvg'];
avg = json['avg'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['student_id'] = this.studentId;
data['stud_name'] = this.studName;
data['stud_profilepic'] = this.studProfilepic;
data['student_email'] = this.studentEmail;
data['student_mobile'] = this.studentMobile;
data['course_name'] = this.courseName;
data['class_code'] = this.classCode;
data['minAvg'] = this.minAvg;
data['avg'] = this.avg;
return data;
}
}
And my database provider class looks like following
class StudentDbProvider implements Source, Cache {
Database db;
StudentDbProvider() {
init();
}
void init() async {
Directory documentsDirectory = await getApplicationDocumentsDirectory();
final path = join(documentsDirectory.path, "students.db");
db = await openDatabase(path, version: 1,
onCreate: (Database newDb, int version) {
newDb.execute("""
CREATE TABLE STUDENTS(
id INTEGER PRIMARY KEY,
status INTEGER,
msg TEXT,
data BLOB
)
""");
});
}
#override
Future<int> clear() {
return db.delete("STUDENTS");
}
#override
Future<StudentModel> fetchStudents(String disciplineId, String schoolId,
String year_id, String lastIndex) async {
final maps =
await db.query("STUDENTS");
if (maps.length > 0) {
return StudentModel.fromDb(maps.first);
}
return null;
}
#override
Future<int> addStudent(StudentModel studentModel) {
return db.insert("STUDENTS", studentModel.toJson(),conflictAlgorithm: ConflictAlgorithm.ignore);
}
}
final studentDbProvider = StudentDbProvider();
Whenever I tried to fetch the data and stored in the database, I get the following error in the console
NoSuchMethodError: The method 'query' was called on null.
Receiver: null
Tried calling: query("STUDENTS")
#0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
The data gets added to the database but I am not able to query the data from the database.

Answer 1

Reducing at minimum your example, this throws the exception The method 'query' was called on null
because fetch is executed before db is properly initialized:
class Database {
Future<int> query() {
return Future.value(1);
}
}
const oneSecond = Duration(seconds: 1);
class Provider {
Database db;
Provider() {
init();
}
void init() async {
db = await Future.delayed(oneSecond, () => Database());
}
Future<int> fetch() {
return db.query();
}
}
main() async {
var provider = Provider();
await provider.fetch();
}
The problem resides in calling an async method inside a constructor, see also:
Calling an async method from component constructor in Dart
This works:
class Database {
Future<int> query() {
return Future.value(1);
}
}
const oneSecond = Duration(seconds: 1);
class Provider {
Database db;
Provider() {
//init();
}
void init() async {
db = await Future.delayed(oneSecond, () => Database());
}
Future<int> fetch() {
return db.query();
}
}
main() async {
var provider = Provider();
await provider.init();
await provider.fetch();
}
Please note that init must be awaited, otherwise you will catch the same The method 'query' was called on null.

Answer 2

the problem is the init must be awaited. here what I did to fix it
_onCreate(Database db, int version) async {
await db.execute('CREATE TABLE ... <YOUR QUERY CREATION GOES HERE>');
}
Future<Database> getDatabaseInstance() async {
final String databasesPath = await getDatabasesPath();
final String path = join(databasesPath, '<YOUR DB NAME>');
return await openDatabase(path, version: 1, onCreate: _onCreate);
}
Future<int> save(Contact contact) {
return getDatabaseInstance().then((db) {
final Map<String, dynamic> contactMap = Map();
contactMap['name'] = contact.name;
contactMap['account_number'] = contact.accountNumber;
return db.insert('contacts', contactMap);
});
}
The SQFlite page gives a good example about it and helps a lot.
https://github.com/tekartik/sqflite/blob/master/sqflite/doc/opening_db.md