Why converting a Firestore querySnapshot into custom objects with compactMap returns empty although the querySnapshot contains documents? - swift

Question

Screenshot of a Firestore Document
I am using Swift, Xcode and a Firestore database.
I created a TableView and a Custom Object Class (MediumSample) with a dictionary and want to load my Firestore documents and show them in the TableView.
The documents (example in the screenshot) are loaded from Firestore correctly but the conversion into the object did not work. The list of objects returned from compactMap is always empty.
Here is my code snippets. It would be great, if someone has a hint on what is going wrong.
Custom Object Class (simplified):
import Foundation
import FirebaseFirestore
protocol MediumSampleDocumentSerializable {
init?(dictionary: [String:Any])
}
struct MediumSample {
var id: String
var field_t: String
var field_i: Int64
var field_b1: Bool
var field_b2: Bool
var field_g: FirebaseFirestore.GeoPoint
var field_d: Date
var field_a: [String]
var usecase: String
var dictionary: [String:Any] {
return [
"id": id,
"field_t": field_t,
"field_i": field_i,
"field_b1": field_b1,
"field_b2": field_b2,
"field_g": field_g,
"field_d": field_d,
"field_a": field_a,
"usecase": usecase
]
}
}
extension MediumSample: MediumSampleDocumentSerializable {
init?(dictionary: [String:Any]) {
guard let id = dictionary ["id"] as? String,
let field_t = dictionary ["field_t"] as? String,
let field_i = dictionary ["field_i"] as? Int64,
let field_b1 = dictionary ["field_b1"] as? Bool,
let field_b2 = dictionary ["field_b2"] as? Bool,
let field_g = dictionary ["field_g"] as? FirebaseFirestore.GeoPoint,
let field_d = dictionary ["field_d"] as? Date,
let field_a = dictionary ["field_a"] as? [String],
let usecase = dictionary ["usecase"] as? String else {return nil}
self.init (id: id, field_t: field_t, field_i: field_i, field_b1: field_b1, field_b2: field_b2, field_g: field_g, field_d: field_d, field_a: field_a, usecase: usecase)
}
}
Declaration of the database and array and calling the loading function:
import UIKit
import FirebaseFirestore
class MediumTableViewController: UITableViewController {
//MARK: Properties
var db: Firestore!
var mediumsamples = [MediumSample]()
override func viewDidLoad() {
super.viewDidLoad()
db = Firestore.firestore()
loadMediumSamples()
}
Function for loading the Firestore documents to fill the Array:
private func loadMediumSamples() {
//run the Firestore query
db.collection(Constants.MEDIUMS).whereField("usecase", isEqualTo: Constants.USECASE)
.getDocuments() { querySnapshot, err in
if let err = err {
print("Error getting documents: \(err)")
} else {
//initialise an array of medium objects with Firestore data snapshots
self.mediumsamples = querySnapshot!.documents.compactMap({MediumSample(dictionary: $0.data())})
//fill the tableView
DispatchQueue.main.async {
self.tableView.reloadData()
print(self.mediumsamples)
}
print("Mediums List", self.mediumsamples) // This line returns: Mediums List []
print("Mediums List size", (self.mediumsamples.count)) // This line returns: Mediums List size 0
for document in querySnapshot!.documents {
print("\(document.documentID) => \(document.data())") // This line returns the snapshot documents correctly!
}
}
}
}
Here is how the screenshot object object is added:
func addMediumSamples() {
let currentDateTime = Date()
let location = FirebaseFirestore.GeoPoint(latitude: 0, longitude: 0)
let mediumsample = MediumSample(id: "an id", field_t: "field_t", field_i: 10, field_b1: true, field_b2: false, field_g: location, field_d: currentDateTime, field_a: [Constants.SAMPLE_DEV], usecase: Constants.SAMPLE_DEV)
var ref: DocumentReference? = nil
ref = self.db.collection(Constants.MEDIUMS).addDocument(data: mediumsample.dictionary) {
error in
if let error = error {
print("Error writing city to Firestore: \(error)")
} else {
print("Document added with id : \(ref!.documentID)")
}
}
}

Answer 1

The problem is in the MediumSample struct, in the field_d type (Date).
The type of that field in your Cloud Firestore database is Timestamp.
The field "field_d" in the MediumSample struct expects a value of type Date.
You can change the type to the FirebaseFirestore.Timestamp, or you can convert it to Date when mapping and before passing to the MediumSample.
eg. for converting Timestamp to Date in Swift
let date = timestamp.dateValue()