React Native If Exist in Firebase - reactjs

Question

My firebase structure:
Hello, i am trying;
One person opening a announcement with own userid. After another person get in this announcement's inside. If second person wants to send a notification to first person, he uses first person's userid's table in firebase and he put own userid's to first persons userid's table. If first person wants to see, who sends notification to him, he looks to own user id's table and he sees other peoples userids (other people are sended notification to him).. Now one person can send lots of time notification, i want to one person can send just one time for this reason i am trying to control it.
if (firebase.database().ref(`/bavuruistek/${userid}`).child(katilan) === null ) {
firebase.database().ref(`/bavuruistek/${userid}`)
.push({
katilan, istek })
.then(() => {
dispatch({ type: STUDENT_REQUEST_SUCCESS });
Actions.pop();
});
}
if (firebase.database().ref(`/bavuruistek/${userid}`).child(katilan) !== null) {
console.log(firebase.database().ref(`/bavuruistek/${userid}/${katilan}`));
Alert.alert(
'Mesaj',
'Daha önce başvurunuz yapılmış!',
[
{ text: 'Tamam', onPress: () => null }
]
);
}

Answer 1

firebase.database().ref(`/basvuruistek/${userid}`).on('value', (snapshot) => {
const { currentUser } = firebase.auth();
const notes = snapshot.val();
Object.keys(notes).forEach(key => {
if (notes[key].katilan !== currentUser.uid) {
this.setState({ click: true });
} if (notes[key].katilan === currentUser.uid) {
this.setState({ click: false });
}
});
});

Answer 2

You are trying to use firebase.database().ref(`/bavuruistek/${userid}`).child(katilan) inside an if statement, which doesn't work because 1) .child() returns a Reference object. Trying something like this may help you achieve the intended outcome:
firebase.database().ref(`/bavuruistek/${userid}`).child(katilan).once('value').then(snapshot => {
if (snapshot.val() === null ) {
firebase.database().ref(`/bavuruistek/${userid}`)
.update({
katilan, istek
})
.then(() => {
dispatch({ type: STUDENT_REQUEST_SUCCESS });
Actions.pop();
});
} else {
console.log(snapshot.val());
Alert.alert(
'Mesaj',
'Daha önce başvurunuz yapılmış!',
[
{ text: 'Tamam', onPress: () => null }
]
);
}
}
The main difference is that I'm using .once, reading the resulting snapshot value, and using that as part of the if statement to check if it is null.