DocumentReferenceExample.swift

import UIKit
import RxSwift
import FirebaseFirestore

class ViewController: UIViewController {
    @IBOutlet weak var tableView: UITableView!
    var posts = [Post]()
    let disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()
        Firestore.firestore().collection("posts").addSnapshotListener { [unowned self] (snapshot, error) in
            guard let snapshot = snapshot else { return }
            var posts = snapshot.documents.map {
                try! Post(key: $0.documentID, dictionary: $0.data())
            }

            let observables: [Observable<User>] = posts.map {
                self.observeUser(documentRef: $0.userReference)
            }

            Observable.zip(observables).subscribe({
                guard let users = $0.element else { return }
                for user in users {
                    for (index, post) in posts.enumerated() where post.userReference.documentID == user.id {
                        posts[index].userEntity = user
                    }
                }
                self.posts = posts
                self.tableView.reloadData()
            }).disposed(by: self.disposeBag)
        }
    }

    fileprivate func observeUser(documentRef: DocumentReference) -> Observable<User> {
        return Observable.create { observer in
            documentRef.addSnapshotListener { snapshot, error in
                guard let snapshot = snapshot else {
                    observer.on(.error(error!))
                    return
                }
                do {
                    observer.on(.next(try User(key: snapshot.documentID, dictionary: snapshot.data())))
                } catch {
                    observer.on(.error(error))
                }
            }
            return Disposables.create()
        }
    }
}

extension ViewController: UITableViewDelegate, UITableViewDataSource {
    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return posts.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: .value2, reuseIdentifier: nil)
        cell.textLabel?.text = posts[indexPath.row].userEntity?.name
        cell.detailTextLabel?.text = posts[indexPath.row].title
        return cell
    }
}

struct Post {
    let id: String
    let title: String
    let userReference: DocumentReference
    var userEntity: User?

    init(key: String, dictionary: [String: Any]) throws {
        guard let title = dictionary["title"] as? String,
            let userReference = dictionary["userReference"] as? DocumentReference else { throw FirestoreError.initializeFailed }

        self.id = key
        self.title = title
        self.userReference = userReference
    }
}

struct User {
    let id: String
    let name: String

    init(key: String, dictionary: [String: Any]) throws {
        guard let name = dictionary["name"] as? String else { throw FirestoreError.initializeFailed }
        self.id = key
        self.name = name
    }
}

enum FirestoreError: Error {
    case initializeFailed
    var localizedDescription: String {
        switch self {
        case .initializeFailed:
            return "error"
        }
    }
}