firebase Starting point was already set - node.js

Question

I use firebase admin and realtime database on node.js
Data look like
When I want to get data where batch = batch-7, I was doing
let batch = "batch-7";
let ref = admin.database().ref('qr/');
ref.orderByChild("batch").equalTo(batch).on('value', (snapshot) =>
{
res.json(Object.assign({}, snapshot.val()));
ref.off();
});
All was OK!
But now i should create pagination, i.e. I should receive data on 10 elements depending on the page.
I use this code:
let page = req.query.page;// num page
let batch = req.params.batch;// batch name
let ref = admin.database().ref('qr/');
ref.orderByChild("batch").startAt(+page*10).limitToFirst(10).equalTo(batch)
.on('value', (snapshot) =>
{
res.json(Object.assign({}, snapshot.val()));
ref.off();
});
But I have error:
Query.equalTo: Starting point was already set (by another call to startAt or equalTo)
How do I get data in the amount of N, starting at position M, where batch equal my batch

Answer 1

You can only call one startAt (and/or endAt) OR equalTo. Calling both is not possible, nor does it make a lot of sense.
You seem to have a general misunderstanding of how startAt works though, as you're passing in an offset. Firebase queries are not offset based, but work purely on the value, often also referred to as an anchor node.
So when you want to get the data for a second page, and you order by batch, you need to pass in the value of batch for the anchor node; first item that you want to be returned. This anchor node is typically the last item of the previous page, since you don't know the first item of the next page yet. And for this anchor node, you need to know the value of the item you order on (batch) and usually also its key (if/when there may be multiple nodes with the same value for batch).
It also means that you usually request one item more than you need, which is the anchor node.
So when you request the first page, you should track the key/batch of the last node:
var lastKey, lastValue;
ref.orderByChild("batch").equalTo(batch).limitToFirst(10).on('value', (snapshot) => {
snapshot.forEach((child) => {
lastKey = child.key;
lastValue = child.child('batch').value();
})
})
Then when you need the second page, you do a query like that:
ref.orderByChild("batch").start(lastValue, lastKey).endAt(lastValue+"\uf8ff").limitToFirst(11).on('value', (snapshot) => {
snapshot.forEach((child) => {
lastKey = child.key;
lastValue = child.child('batch').value();
})
})
There's one more trick above here: I use startAt instead of equalTo, so that we can get pagination working. But it then uses endAt to ensure we still end at the correct item, by using the last known Unicode character as the last batch value to return.
I'd also highly recommend checking out some of the previous questions on pagination with the Firebase Realtime Database.