Limiting Parse query without query.limit in Swift - ios

Question

I need to do two things in the same Parse query. 1) I need to find the total number of objects returned by the given query; and 2) Only display the first 20 objects. I can't do both by setting query.limit = 20because the total number of objects will only be 20. If the total number of objects is 100, I need to get that number.
So, how can I progammatically display only the first 20 objects while still receiving all 100?
var query = PFQuery(className: "Professions")
query.whereKey("user", equalTo: PFUser.currentUser()!.username!)
query.orderByDescending("createdAt")
// query.limit = 20
query.findObjectsInBackgroundWithBlock({ (objects, error) -> Void in
if error == nil {
if let objects = objects as? [PFObject] {
for object in objects {
// I tried using something like:
// for var i = 0; i <= 20; i++ {
// if object[i] {
// But get 'Int' is not convertible to 'String'
if let title = object["title"] as? String {
println(title)
}
}
}
} else {
println(error)
}
})
When I try setting the following, I always get fatal error: array index out of range.
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 20
}

Answer 1

Maybe not the most elegant solution, but i think you need to do two querys on the same query. One for the object.count and one with query.limit.
var query = PFQuery(className: "Professions")
query.whereKey("user", equalTo: PFUser.currentUser()!.username!)
query.orderByDescending("createdAt")
query.findObjectsInBackgroundWithBlock({ (objects, error) -> Void in
if error == nil {
if let objects = objects as? [PFObject] {
var numberOfObjects = objecs.count
}
else {
println(error)
}
query.limit = 20
query.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]?, error: NSError?) -> Void in
if error == nil {
if let objects = objects as? [PFObject] {
for object in objects {
if let title = object["title"] as? String {
println(title)
}
}
}
else {
println(error)
}
}
})

Answer 2

First, if you just need to count objects, Parse has a method for that:
query.countObjectsInBackgroundWithBlock
Then you can issue another PFQuery to get the first 20 objects. Fetching all the objects just to count them locally is bad design.
Nonetheless, if you still have good reason to retrieve all objects (limited by Parse at 1000) and process them locally, getting the first 20 is not done with Parse, it's done in Swift, locally, after you have fetched all objects.
var fetchedProfessions = [PFObject]()
var query = PFQuery(className: "Professions")
query.whereKey("user", equalTo: PFUser.currentUser()!.username!)
query.orderByDescending("createdAt")
query.limit = 100
query.findObjectsInBackgroundWithBlock({ (objects, error) -> Void in
if error == nil {
if let objects = objects as? [PFObject] {
// Capture your results
self.fetchedProfessions = objects
}
} else {
print(error)
}
})
// Get the first 20
let firstTwentyProfessions = retrievedObjects[0..19]