How to check whether a user exists in firebase cloudstore or not in a flutter application while reigisteration is done? - flutter

Question

When i try to check whether a particular number exists in firebase collection or not I always get a null in return. What is the reason for that?
How can I overcome this issue so that the function _numberCheck() returns a Boolean value which I can use for further processing?
The code for the registration page is as follows:
Future<bool> _numberCheck(mobileno) async {
print("Start of the function");
db
.collection("transcriber_user_registeration")
.getDocuments()
.then((QuerySnapshot snapshot) {
snapshot.documents.forEach((f) async {
isPresent = true;
isPresent = false;
if ('${f.data['userid']}' == mobileno) {
setState(() {
isPresent = true;
});
} else {
setState(() {
isPresent = false;
});
}
});
});
// print(isPresent);
return isPresent;
}

Answer 1

For your case, you can try to use a query that only returns values that are not null. This way, in case it's not null, it will return true and if it's null, it will return false.
One example of how to achieve that - as better explained in this other question from the Community here - you can use the function hasAll(). This function returns only if all values are present. So, you would use it to load all the values to array, verifying this way, if they are not null - something like the below example.
!resource.data.keys().hasAll(['mobileno'])
There are other examples, such as using a String to compare values using the where() function, so you won't get the null values. More information on doing queries are available in this link.
I would recommend you to take a look at them and give these above options a try, to verify if they will help you. :)
Let me know if the information helped you!

Answer 2

You function returns null because you are returning isPresent without setting it to true, you only set isPresent = false, which means isPresent will be null otherwise, as variables hold objects references, so it will hold null if you didn't initialize it.
bool isPreset; // isPreset is null
isPreset = true; // isPreset is true
isPreset = false; // isPreset is false
If a variable lacks a value, it's value is null. Always initialize your variables to something.
Also, if f.data['userid'] is a String, you don't have to make it a String again by surrounding it with '${}'
Change your code to:
Future<bool> _numberCheck(mobileno) async {
QuerySnapshot snapshot = await db.collection("transcriber_user_registeration")
.getDocuments();
snapshot.documents.forEach((f) {
if (f.data['userid'] == mobileno) {
return false;
}
});
return true;
}
And call setState for the result of this function where you call it. Or just use a FutureBuilder to deal directly with the result and refresh your screen automatically.
If it returns true it means the query collection result is an empty collection or the if (f.data['userid'] == mobileno) is never true. So check the values on both.