Querying from Parse - ios

Question

Basically I am trying to display 3 users, I am querying from the _User class the following: username, profilePicture & Name.
After that I would like to query the last photo they posted from the Posts class.
Here is how I have setup my code :
let userQuery = PFQuery(className: "_User")
userQuery.limit = 3
userQuery.addDescendingOrder("createdAt")
userQuery.findObjectsInBackgroundWithBlock ({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
self.profilePicArray.removeAll(keepCapacity: false)
self.usernameArray.removeAll(keepCapacity: false)
self.fullnameArray.removeAll(keepCapacity: false)
self.uuidArray.removeAll(keepCapacity: false)
for object in objects! {
self.profilePicArray.append(object.valueForKey("profilePicture") as! PFFile)
self.usernameArray.append(object.valueForKey("username") as! String)
self.fullnameArray.append(object.valueForKey("firstname") as! String)
self.uuidArray.append(object.valueForKey("uuid") as! String)
}
let imageQuery = PFQuery(className: "Posts")
imageQuery.whereKey("username", containedIn: self.usernameArray)
imageQuery.findObjectsInBackgroundWithBlock({ (objects:[PFObject]?, error:NSError?) -> Void in
if error == nil {
self.lastPicArray.removeAll(keepCapacity: false)
for object in objects! {
self.lastPicArray.append(object.valueForKey("image") as! PFFile)
}
self.collectionView.reloadData()
} else {
print(error!.localizedDescription)
}
})
} else {
print(error!.localizedDescription)
}
})
}
But when I run it, it isn't showing the correct image for the user showing...I cannot get my head round it, it's been driving me mad for several hours now!!

Answer 1

The background thread isn't a problem for fetching your images, but your image query is not set to return images in any specific order, nor does it seem to limit how many photos will come back for each user (unless you have it set up so that the Post object can only hold one post per user). You could handle this in a few different ways, but the easiest way might be to sort your object array in some predictable order, using the username as the sort key, and then when you get your images back you can sort them in the same order based on their username property before putting them into the array that drives the collectionView.
One other note - you'll want to put your .reloadData() call back on the main thread or it'll happen at an unpredictable time.

Answer 2

It might cause by data transferring in different time cost since you use findObjectsInBackgroundWithBlock. Threads running in background are hard to control. So the order of images in imageQuery objects is different with usernameArray. Or findObjectsInBackgroundWithBlock doesn't guarantee that data will be fetched in same order with keys set. Not 100% sure about this.
But try putting the image fetching procedure inside the first for object in objects! loop. It might be able to solve your problem.
For sorting answer from creeperspeak, you should be able to get username back from imageQuery class just like object.valueForKey("username") in your first loop. Then match them with self.usernameArray.