Error using Codable to map user data in swift - ios

Question

I am trying to replace manual mapping when fetching a user by using Codable. In doing so, I am getting some errors. The errors I am getting are 'cannot find self in scope'.
Here is my fetchUser function:
import SwiftUI
import Firebase
import FirebaseFirestoreSwift
func fetchUser(documentId: String) {
let db = Firestore.firestore()
let docRef = db.collection("Users").document(documentId)
docRef.getDocument { document, error in
if let error = error as NSError? {
self.errorMessage = "Error getting document: \(error.localizedDescription)"
}
else {
if let document = document {
do {
self.user = try document.data(as: UserModel.self)
}
catch {
print(error)
}
}
}
}
}
Here is my UserModel:
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 favouriteProducts: [FavourtieProducts]
}
What's going wrong here?
The following is how I did it before. This worked but when I added more variables to the UserModel I wanted to change how it is fetched to make the code better.
import SwiftUI
import Firebase
import FirebaseFirestoreSwift
let ref = Firestore.firestore()
func fetchUser(uid: String,completion: #escaping (UserModel) -> ()){
let db = Firestore.firestore()
#EnvironmentObject var sharedData: SharedDataModel
ref.collection("Users").document(uid).getDocument { (doc, err) in
guard let user = doc else{return}
let username = user.data()?["username"] as? String ?? "No Username"
let pic = user.data()?["imageurl"] as? String ?? "No image URL"
let bio = user.data()?["bio"] as? String ?? "No bio"
let uid = user.data()?["uid"] as? String ?? ""
let isVerified = user.data()?["isVerified"] as? Bool ?? false
DispatchQueue.main.async {
completion(UserModel(username: username, pic: pic, bio: bio, uid: uid, isVerified: isVerified))
}
}
}
How do I turn the first(new one) fetchUser function from a global function to a local function?