Query items between x and y with Firebase - ios

Question

So, I am using the official Hacker News API and it is hosted on Firebase.
The problem I am having is that I want to get a subset of a list basically.
Something along the way of.
let topNewsRef = firebase.childByAppendingPath("topstories").queryLimitedToFirst(UInt(batchSize + offset)).queryLimitedToLast(UInt(batchSize))
[I know this does not work but I would like this effect]. Basically I want a subset of a set, specified by a range; from item 2 to item 15, for example].
Say I want 50 items from the 75th item, but the above is not working. So the question is; how do I achieve the same effect?
For example; given a list of 100 items in Firebase. I want all item from the 50th and 75th. There is no property that gives away the order of the items.
Here is my current solution;
let topNewsRef = firebase.childByAppendingPath("topstories").queryLimitedToFirst(UInt(batchSize + offset))
var handle: UInt?
handle = topNewsRef.observeEventType(.Value) { (snapshot: FDataSnapshot!) -> Void in
if let itemIDs = snapshot.value as? [Int] {
itemIDs.dropFirst(offset) // This drops all items id I already fetched ...
for itemID in itemIDs {
let itemRef = self.firebase.childByAppendingPath("item/\(itemID)")
var itemHandle: UInt?
itemHandle = itemRef.observeEventType(.Value, withBlock: { (snapshot: FDataSnapshot!) -> Void in
if let itemHandle = itemHandle {
itemRef.removeObserverWithHandle(itemHandle)
}
if let json = snapshot.value as? [String:AnyObject],
// Handle JSON ...
}
})
}
}
if let handle = handle {
topNewsRef.removeObserverWithHandle(handle)
}
} // offset += batchSize
... which is gettting all items from the start (offset) to the end (batchSize + offset), then I drop the first end of the list by the size of offset. Thus leaving on the list with a size of batchSize left.

Answer 1

Following our comments to your question,
You can use .queryOrderedByKey in combination with queryStartingAtValue:, queryEndingAtValue:
The data in /topstories is stored as an array. when ordering by key, the indices of the objects in the array are the keys. Then, all you need to do is cast the offset and batchSize as strings and pass them to queryStartingAtValue: & queryEndingAtValue: like so:
ref.queryOrderedByKey().queryStartingAtValue(String(offset)).queryEndingAtValue(String(offs‌​et+batchSize-1));
Don't forget to subtract 1 for the endingValue if you are incrementing offset by batchsize.
For the nth query in the series,
startingAtIndex = initialOffset + (batchSize * (n - 1))
endingAtIndex = initialOffset + (batchSize * n) - 1
For example, let batchSize=100 and the initial offset=0.
The first 100 items are keys 0 through 99. The key (index) of the first item in the array is 0.
If you query for endingAt(offset+batchSize), you will be ending at index 100, which is the 101st item in the array.
Thus, you will be getting 101 items -- one more than your batch size.
the second query will then be for items with indices 100-199 (starting at 0+batchsize and ending at 0+batchsize+batchsize-1), which is 100 items.
Here is an example PLNKR of the same concept implemented in JavaScript.
:)

Answer 2

Okay, so the problem was solved with the following query.
firebase.childByAppendingPath("topstories").queryOrderedByKey().queryStartingAtValue(String(offset)).queryEndingAtValue(String(offset + batchSize - 1))
The thing is that the Firebase documentation does not specify that the "AnyObject!" parameters can be used as indices on the query. They also forget to mention that the indices should be of type String.