Pagination in firebase with identical child values - ios

Question

My data structure is as follows:
users:
user1:
-carModel: evo x
-username: importguy
-region: north east
user2:
-carModel: evo x
-username: evoguy
-region: north east
user3:
-carModel: mustang gt
-username: muscleguy
-region: south east
I want the user to be able to search for a car, say evo, and display results of users who own those particular cars. I need to paginate these results for my app. The problem is I can't figure out how to properly query this. Here is what i have so far.
func fetchUsersBy(car: String) {
if self.carCurrentKey == nil {
let ref = USER_REF.queryOrdered(byChild: "carModel").queryStarting(atValue: car).queryLimited(toFirst: 3)
ref.observeSingleEvent(of: .value, with: { (snapshot) in
guard let snap = snapshot.children.allObjects as? [FIRDataSnapshot] else { return }
guard let last = snapshot.children.allObjects.last as? FIRDataSnapshot else { return }
snap.forEach({ (snapshot) in
guard let userDict = snapshot.value as? Dictionary<String, AnyObject> else { return }
guard let carModel = userDict["carModel"] as? String else { return }
if carModel.contains(car) {
print(snapshot)
}
})
self.carCurrentKey = last.key
self.carCurrentValue = last.childSnapshot(forPath: "carModel").value as? String
})
} else {
// where to start next query?
let ref = USER_REF.queryOrder(byChild: "carModel").queryStarting(atValue: self.carCurrentValue)
}
}
I have to order the query by carModel, in order to group all of the users with that particular car type together in a snapshot. Since all the car models are the same value, I cannot figure out where to start or end the next query for the pagination. Using the reference i have in the else block starts the query at the same place as the block above. Any help or advice would be much appreciated.
I considered doing a fan out, and making a separate structure for car types. This would be difficult though.

Answer 1

For both startAt() and endAt(), you can pass a second value, childKey as shown here.
So your query will look something like this:
let ref = USER_REF.queryOrdered(byChild: "carModel").queryStarting(atValue: self.carCurrentValue, childKey: self.carCurrentKey).queryLimited(toFirst: 3+1)
Note that I used toFirst: 3+1. That's because, annoyingly, startAt() is inclusive and there's no way to skip the first record. So, since we started from the last record retrieved on the previous page, you will want to query for one extra record and discard the first result.
Here's a more complete example in JavaScript. Not familiar enough to translate this to Swift, but it should give you the algorithm in completion.
class Cursor {
constructor(baseRef, pageSize) {
this.baseRef = baseRef;
this.lastKey = null;
this.lastValue = null;
this.pageSize = pageSize;
}
next() {
let ref = this.baseRef;
if( this.lastValue !== null ) {
// a previous page has been loaded so get the next one using the previous value/key
// we have to start from the current cursor so add one to page size
ref = ref.startAt(this.lastValue, this.lastKey).limitToFirst(this.pageSize+1);
}
else {
// this is the first page
ref = ref.limitToFirst(this.pageSize);
}
return ref.once('value').then(snap => {
const keys = [];
const data = []; // store data in array so it's ordered
snap.forEach(ss => {
data.push(ss.val());
keys.push(ss.key);
});
if( this.lastValue !== null ) {
// skip the first value, which is actually the cursor
keys.shift();
data.shift();
}
// store the last loaded record
if( data.length ) {
const last = data.length - 1;
this.lastKey = keys[last];
this.lastValue = data[last].author;
}
return data;
});
}
}
And here's a working fiddle.
Keep in mind that this is a realtime data stream. So pagination is tricky. It's generally easier to just do infinite scroll than to try and maintain a realistic cursor on a moving data set (records can reorder when data changes, get deleted, added in the middle, etc).