Flutter firestore check if document with specific id exists - firebase

Question

I want to check if the firestore document with a specific id exists. Actually, my idea is to store and fetch user-specific data from firestore. If there is no user-specific data uploaded it will show "no data to show" in the app. Till now what I have done is adding data to firestore with document id equals to the current user's UID.
FirebaseFirestore.instance.collection("doc_folder").doc(currentuser!.uid).set(data);
now I am unable to check if the firestore database contains any document with this data. By far, I have reached:
class Crud {
getData() {
return FirebaseFirestore.instance
.collection('doc_folder')
.where("userId", isEqualTo: currentUser!.uid)
.get();
}
}
#override
void initState() {
crud.getData().then((result) {
snap = result;
setState(() {});
});
super.initState();
}
Widget build(BuildContext context){
return Container(
snap != null? //if code
: //else code
)
}
The above code returns "document exists" even if the data does not exist with the current user's UID.

Answer 1

The following line from your code returns a QuerySnapshot which is not nullable:
FirebaseFirestore.instance.collection('doc_folder').where("userId", isEqualTo: currentUser!.uid).get()
and you assign the returned value of QuerySnapshot to snap and perform the following:
snap != null ? ... : ...
However, this condition will be true regardless of whether a document exists.
You should instead check docs property from QuerySnapshot, which is a list of the document snapshots returned for your query.
I'd prefer the widget of FutureBuilder over calling getData() in initState for this example to make it clear:
#override
Widget build(BuildContext context) {
return FutureBuilder<QuerySnapshot>(
future: crud.getData(),
builder: (context, snapshot) {
if (snapshot.hasData) {
if (snapshot.data.docs.isNotEmpty) {
// Document exists
} else {
// Document does not exist
}
} else {
// Show a loading widget
// ...
}
},
);
}
Full Example
class Crud {
Future<QuerySnapshot> getData() async {
return await FirebaseFirestore.instance
.collection('doc_folder')
.where("userId", isEqualTo: currentUser!.uid)
.get();
}
}
class MyPage extends StatefulWidget {
#override
_MyPageState createState() => _MyPageState();
}
class _MyPageState extends State<MyPage> {
final crud = Crud();
#override
Widget build(BuildContext context) {
return FutureBuilder<QuerySnapshot>(
future: crud.getData(),
builder: (context, snapshot) {
if (snapshot.hasData) {
final querySnaphost = snapshot.data; // Get query snapshot
if (querySnaphost.docs.isNotEmpty) {
// Document exists
final documentSnapshot =
querySnaphost.docs.first; // Get document snapshot
return Text('${documentSnapshot.data()}'); // Get the data
} else {
// Document does not exist
return Text('Document does not exist.');
}
} else {
// Show a loading widget
return CircularProgressIndicator();
}
},
);
}
}

Answer 2

The reason is that value of snapshot is not null even though document doesn't exists. So use below code:-
QuerySnapshot snap=await FirebaseFirestore.instance.collection('doc_folder').where("UserId", isEqualTo: currentuser!.uid).get();
if(snap.docs.isNotEmpty)
DocumentSnapshot doc=snap.docs.first;
print(doc['username']);//like this you can access data
else{
print("Doc doesn't exits");
}

Answer 3

You can do that
var
doc=FirebaseFirestore.instance
.collection('doc_folder')
.where("userId", isEqualTo:
currentUser!.uid)
.get();
if(doc.exist){
print('exist');
}

Answer 4

There is a slight problem with your code. build() is called while async crud.getData() is still running. Therefore snap will be have its default value. If snap's default value is not null, then snap != null will be true and you might assume your snap has its intended value.
Full working code
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
class MyHomePage extends StatefulWidget {
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
Crud crud = Crud();
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Title'),
),
body: FutureBuilder<QuerySnapshot<Map<String, dynamic>>>(
future: crud.getData(),
// above is called everytime the widget is rebuilt which is not optimal
builder: (BuildContext context,
AsyncSnapshot<QuerySnapshot<Map<String, dynamic>>> snapshot) {
if (snapshot.hasError) return Text('Something went wrong');
if (snapshot.connectionState == ConnectionState.waiting)
return CircularProgressIndicator();
if (snapshot.data.docs.isNotEmpty) {
print('document exists');
print(snapshot.data.docs.map((e) => e.data()));
return Container();
} else {
print('document does not exist');
return Container();
}
},
),
);
}
}
class Crud {
Future<QuerySnapshot<Map<String, dynamic>>> getData() {
return FirebaseFirestore.instance
.collection('users')
.where("userId", isEqualTo: 'c1fG8zo0OWgHsPClEKWN')
.get();
}
}

Answer 5

FirebaseFirestore.instance
.collection('Company')
.doc('1235')
.get()
.then((value) {
if (value.exists) {
// Do something Here
}
The question asked to check if the document exists, and the answer accepted only works if docId is stored as field in that doc. Needs Review.