Map-Array.swift

extension Mapper {
    func mapArray(snapshot: DataSnapshot) -> [N] {
        return snapshot.children.map { (child) -> N? in
            if let childSnap = child as? DataSnapshot {
                return N(snapshot: childSnap)
            }
            return nil
        //flatMap here is a trick
        //to filter out `nil` values
        }.flatMap { $0 }
    }
}

func getPosts(completion: @escaping (([Post]) -> Void)) {
    let postRef = Database.database().reference()
        .child("posts")
    postRef.observeSingleEvent(of: .value) { (snapshot) in
        completion(Mapper<Post>().mapArray(snapshot: snapshot))
    }
}