Swift Mapping Custom Object Using Codable - ios

Question

I am trying to practice mapping custom objects using swift & replace all manual mapping by using Codable. This concept is new to me but seems very worthwhile. In this project I want to fetch the user and make sure that all their data is stored in their User Document on firebase (including the custom object 'FavouriteItems'.
My User Model struct is:
import SwiftUI
import FirebaseFirestoreSwift
struct UserModel: Identifiable, Codable{
#DocumentID var id: String?
var username : String
var pic : String
var bio: String
var uid : String
var isVerified: Bool
var favouriteItems: [FavouriteItems]
}
My struct for an Item is:
import SwiftUI
import FirebaseFirestoreSwift
import Firebase
struct Item: Identifiable, Codable {
#DocumentID var id: String?
var item_name: String
var item_type: String
var item_image: String
var item_details: String
var item_uid : String
var didFavourite: Bool? = false
var isFavourite: Bool = false
}
My struct for FavouriteItems is:
import SwiftUI
import FirebaseFirestoreSwift
//MARK: Make Hashable (See Tag Example)?
struct FavouriteItems: Identifiable, Codable {
#DocumentID var id: String?
var item: Item
}
My fetch user function is (edited):
func fetchUser(uid: String, completion: #escaping (UserModel) -> ()){
let db = Firestore.firestore()
let docRef = db.collection("Users").document(uid).getDocument(as: UserModel.self) { result in
switch result {
case .success(let user):
// A `UserModel` value was successfully initialized from the DocumentSnapshot.
print("UserModel: \(user)")
DispatchQueue.main.async {
completion(user)
}
case .failure(let error):
// A `UserModel` value could not be initialized from the DocumentSnapshot.
print("Error decoding UserModel: \(error)")
// handle errors todo
}
}
}
I am getting errors in the fetchUser code including:
Cannot convert value of type 'UserModel.Type' to expected argument type 'FirestoreSource'
Contextual closure type '(DocumentSnapshot?, Error?) -> Void' expects 2 arguments, but 1 was used in closure body
Incorrect argument label in call (have 'as:_:', expected 'source:completion:')
All of these errors occur on the line:
let docRef = db.collection("Users").document(uid).getDocument(as: UserModel.self) { result in
Here is my Podfile: