Populating tableView with data obtained from Firebase - ios

Question

Here is my scenario:
1) datasource to my tableView is an array called books - var books:[BookItem]
2) I populate books anytime some data in db has changed
3) I create BookItem by joining data from two tables, since BookItem consists of some user-specific data and some general data about the book itself.
I wrote a method called createDataSource that takes care of populating books array with BookItems
func createDataSource()
{
self.books.removeAll()
let userId:String = UserDefaults.standard.value(forKey: "UserId") as! String
let bookForUserRef = self.ref!.child("users").child(userId).child("userbooks")
bookForUserRef.observe(.value, with: { snapshot in
var i = 0
for element in snapshot.children
{
i = i + 1
let item:FIRDataSnapshot = element as! FIRDataSnapshot
let postDict = item.value as! [String : AnyObject]
let key = item.key
let booksRef = self.ref!.child("books").child(key)
booksRef.observe(.value, with: { snapshot in
let bookItem = (snapshot as! FIRDataSnapshot).value as! [String : AnyObject]
let id = booksRef.key
print(id)
let record = [bookItem["title"] as! String, bookItem["author"] as! String, "0", "\(bookItem["pagesCount"] as! Int)"]
let item = BookItem(title: bookItem["title"] as! String, author: bookItem["author"] as! String, pagesCount: bookItem["pagesCount"] as! Int, currentPage: postDict["currentPage"] as! Int, language: bookItem["language"] as! String, year: nil, ISBN: nil, id: id, added: postDict["added"] as? String ?? "21")
self.books.append(item)
self.tableView.reloadData()
})
}
})
}
Anytime I call this method, all existing elements gets duplicated in books array.
I thought it might have something to do with for loop, but after debugging it seems alright.
I suppose there is something specific with observers in firebase that duplicates the elements. I am rather new to firebase and I suppose my approach might be a bit messed and confusing.
What should be modified in the createDataSource() method, to get rid of elements duplication?
Also, is my approach to working with firebase and tableView correct?

Answer 1

Thanks to #ntoonio comment I got it solved
func createDataSource()
{
let userId:String = UserDefaults.standard.value(forKey: "UserId") as! String
let bookForUserRef = self.ref!.child("users").child(userId).child("userbooks")
bookForUserRef.observe(.value, with: { snapshot in
var i = 0
self.books.removeAll()
for element in snapshot.children
{
i = i + 1
let item:FIRDataSnapshot = element as! FIRDataSnapshot
let postDict = item.value as! [String : AnyObject]
let key = item.key
let booksRef = self.ref!.child("books").child(key)
booksRef.observe(.value, with: { snapshot in
let bookItem = (snapshot as! FIRDataSnapshot).value as! [String : AnyObject]
let id = booksRef.key
print(id)
let record = [bookItem["title"] as! String, bookItem["author"] as! String, "0", "\(bookItem["pagesCount"] as! Int)"]
let item = BookItem(title: bookItem["title"] as! String, author: bookItem["author"] as! String, pagesCount: bookItem["pagesCount"] as! Int, currentPage: postDict["currentPage"] as! Int, language: bookItem["language"] as! String, year: nil, ISBN: nil, id: id, added: postDict["added"] as? String ?? "21")
self.books.append(item)
self.tableView.reloadData()
})
}
})
}