An example of a book model that implements the Model protocol

Book.swift

import Foundation
import FirebaseDatabase

struct Book {
    var id: String?
    var isbn: String
    var title: String
    var authors: String
    var year: String
    var country: String
    var createdAt: Date
}

// MARK: - Model
extension Book: Model {
    var serialized: [String : Any] {
        return [
            "isbn": isbn,
            "title": title,
            "authors": authors,
            "year": year,
            "country": country,
            "createdAt": createdAt.timeIntervalSince1970,
        ]
    }
    
    init?(snapshot: FIRDataSnapshot) {
        guard
            let child = snapshot.value as? [String : Any],
            let isbn = child["isbn"] as? String,
            let title = child["title"] as? String,
            let authors = child["authors"] as? String,
            let year = child["year"] as? String,
            let country = child["country"] as? String,
            let createdAt = child["createdAt"] as? TimeInterval
            else { return nil }
        
        self.id = snapshot.key
        self.isbn = isbn
        self.title = title
        self.authors = authors
        self.year = year
        self.country = country
        self.createdAt = Date(timeIntervalSince1970: createdAt)
    }
}