Firebase infinite loop insert my item on child_added/set - firebase

Question

I got an infinite loop inserting an item to my Firebase, so when I click on my post form, it inserts my item until I kill the process. Can you help me how to solve it?
PS : I'm using VueJS
var usersRef = new Firebase('https://xxxxxxxxxxxxxxxxxx.firebaseio.com/userslist/');
var vm = new Vue({
el: '#list1',
data: function () {
return{
// Initialisation du tableau de users
users: [],
sortKey: 'id',
reverse: 1,
nextKey: null
};
},
ready: function () {
// this works
//this.sortKey = 'name';
},
methods: {
updateUsers: function () {
},
removeUser: function (item) {
usersRef.child(item.id).remove();
},
addItem: function (e) {
e.preventDefault();
// get form data as Json
var jsonData = ConvertFormToJSON('form_add');
//console.log(jsonData);//test ok
//get the last item id and save it to next key
usersRef.limitToLast(1).on('child_added', function (snapshot) {
var lastKey = parseInt(snapshot.key());
this.nextKey = lastKey + 1;
console.log('nextKey ' + nextKey);//test ok
//
// save data to firebase
usersRef.child(this.nextKey).set(jsonData, function (snap) {
//console.log('add success');//test
//Notification par Jquery
var itemAdded = snap.val();
$.notify(itemAdded.firstname + " " + itemAdded.name + " à été ajouté", "success", {position: "top right"});
this.pendingKey = 0;
});
});
},
// Tri des colonnes
sortBy: function (_sortKey) {
this.reverse = (this.reverse == -1) ? 1 : -1;
this.sortKey = _sortKey;
console.log("SortKey " + this.sortKey);
}
}
});
usersRef.on('child_added', function (snapshot) {
var item = snapshot.val();
item.id = snapshot.key();
console.log('id ' + item.id);
vm.users.push(item);
}, function (errorObject) {
console.log("The read failed: " + errorObject.code);
});
thanks for your help

Answer 1

OK I've found a solution :
//get the last item id and save it to next key
var _nextKey = this.nextKey;
usersRef.limitToLast(1).on('child_added', function (snapshot) {
var lastKey = parseInt(snapshot.key());
_nextKey = lastKey + 1;
});
this.nextKey = _nextKey;
console.log('nextKey ' + this.nextKey);//test ok
// save data to firebase
usersRef.child(this.nextKey).set(jsonData, function (snap) {
//console.log('add success');//test
//Notification par Jquery
var itemAdded = snap.val();
$.notify(itemAdded.firstname + " " + itemAdded.name + " à été ajouté", "success", {position: "top right"});
});
May it help someone!