Firebase database transactional search and update - firebase

Question

I have a collection in firebase real time database that is a pool of codes that can be used once per 'store'. I need to search for an unused code, then mark it reserved by a store in an atomic fashion. The problem is I can't figure out how to do a transactional search and update in firebase, and the unused code is being 'used' multiple times until it gets updated.
const getUnusedCode = (storeID) => {
const codeRef = rtdb.ref('codes');
return codeRef
.orderByChild(storeID)
.equalTo(null)
.limitToFirst(1)
.once('child_added')
.then(snap => {
//setting the map {[storeID]:true} reserves the code
return snap.ref.update({ [storeID]: true }).then(() => {
return snap.key;
});
});
};
Edit: Here is the structure of the 'codes' collection:
{
"-LQl9FFD39PAeN5DnrGE" : {
"code" : 689343821901,
"i" : 0,
"5s6EgdItKW7pBIawgulg":true,
"ZK0lFbDnXcWJ6Gblg0tV":true,
"uKbwxPbZu2fJlsn998vm":true
},
"-LQl9FOxT4eq6EbwrwOx" : {
"code" : 689343821918,
"i" : 1,
"5s6EgdItKW7pBIawgulg":true
},
"-LQl9FPaUV33fvkiFtv-" : {
"code" : 689343821925,
"i" : 2
},
"-LQl9FQEwKKO9T0z4LIP" : {
"code" : 689343821932,
"i" : 3,
"ZK0lFbDnXcWJ6Gblg0tV":true
},
"-LQl9FQsEVSNZyhgdHmI" : {
"code" : 689343821949,
"i" : 4,
"5s6EgdItKW7pBIawgulg":true,
"uKbwxPbZu2fJlsn998vm":true
}
}
In this data, "5s6EgdItKW7pBIawgulg" is a store id, and true means this code has been used for this store
When new items are being imported, this function may get called hundres of times a minute, and is returning duplicates since it's not an atomic search-then-update. Is this possible in Firebase?

Answer 1

From what I understand you have a structure like this
codes: {
"code1": {
storeid: "store1"
},
"code2": {
storeid: "store2"
}
}
And you're trying to transactionally update it per store.
If this is the only update you're trying to do, I'd highly recommend inverting your data structure:
codes: {
"store1": "code1",
"store2": "code2"
}
On this structure the transaction for a store is quite simple, since the path is known:
var storeRef = firebase.database().ref("codes").child("store1");
storeRef.transation(function(current) {
if (current) {
// remove the code from the database
return null;
}
else {
// abort the transaction, since the code no longer exists
return undefined;
}
});
If you can't change the data structure, I'd probably user your current code to find the DatabaseReference to the code, and then use a transaction within the callback to update:
codeRef
.orderByChild(storeID)
.equalTo(null)
.limitToFirst(1)
.once('child_added')
.then(snap => {
//setting the map {[storeID]:true} reserves the code
return snap.ref.transaction(function(current) {
if (!current || current[storeId]) {
// the node no longer exists, or it already was claimed for this store
return undefined; // abort the transaction
}
else {
current[storeId] = true;
return current;
}
})
});