Query in Firebase by value of childs [duplicate] - firebase

Question

This question already has answers here:
Firebase search by child value
(2 answers)
Closed 3 years ago.
{
"users" : {
"1320104182" : {
"datas" : {
"bio" : "some_data",
"picture" : "some_data",
"username" : "some_data",
"website" : "some_data",
"followers" : 14,
}
},
"3271376571" : {
"datas" : {
"bio" : "some_data",
"picture" : "some_data",
"username" : "some_data",
"website" : "some_data",
"followers" : 10,
}
}
}
}
I'm new to Firebase and i'm trying to do multiple think here without any success so far.
How can i retrieve a user by his "username" without knowing the key ?
Or how can i order the users by followers ?
I tried everything i could find in the documentation for a few hours i'm desperate.

Answer 1

This seems fairly easy:
var ref = firebase.database().ref("users");
var query = ref.orderByChild("database/username").equalTo("some_data");
query.once("value", function(snapshot) {
snapshot.forEach(function(child) {
console.log(child.key, child.val().bio);
});
});

Answer 2

Firebase is not so good when you need fancy queries. You must handle everything in your client (JavaScript) which is not the best approach when dealing with large data. In this case, I'd suggest you something like this:
const nameToSearch = 'John';
firebase.ref('users').once('value') //get all content from your node ref, it will return a promise
.then(snapshot => { // then get the snapshot which contains an array of objects
snapshot.val().filter(user => user.name === nameToSearch) // use ES6 filter method to return your array containing the values that match with the condition
})
To order by followers, you can either also apply sort() (see example 1) or any of firebase default methods orderByChild() (see example 2), orderByKey (see example 3), or orderByValue (see example 4)
Example 1:
firebase.database().ref("users").once('value')
.then(snapshot => {
const sortedUsers = snapshot.sort((userA, userB) => {
if (userA.name < userB.name) {
return -1;
}
if (userA.name > userB.name) {
return 1;
}
return 0;
})
})
Example 2:
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").on("child_added", function(snapshot) {
console.log(snapshot.key + " was " + snapshot.val().height + " m tall");
});
Example 3:
var ref = firebase.database().ref("dinosaurs");
ref.orderByKey().on("child_added", function(snapshot) {
console.log(snapshot.key);
});
Example 4:
var scoresRef = firebase.database().ref("scores");
scoresRef.orderByValue().limitToLast(3).on("value", function(snapshot) {
snapshot.forEach(function(data) {
console.log("The " + data.key + " score is " + data.val());
});
});
Note: there might be typos in the examples, I wrote just to show you the idea of the concepts.
Check the following docs for more info:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter
https://firebase.google.com/docs/reference/js/firebase.database.Query#orderByChild
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
Hope it helps