Getting conditionals to work within Firebase Observer - ios

Question

I am trying to do a conditional within Firebase Observer.
Essentially I like to check if seat is occupied.
If it is, then orders can be retrieved.
If not then send the restaurant back to the search seat page again.
For some reason, the code within if !taken is never executed even if the condition is met (ie. the owner has inputted the wrong seat number). I have put it within the closure, it should run right?
func retrieveData (){
var taken = false
var seatNumber = "**an Int from other page**"
let refCustomer = Database.database().reference().child("Restaurant").child("Customers")
refCustomer.queryOrdered(byChild: "Seat").queryEqual(toValue: "\(seatNumber)").observeSingleEvent(of: .childAdded, with: { (snapshot) in
if snapshot.exists() {
taken = true
let snapshotValue = snapshot.value as? [String : AnyObject] ?? [:]
self.customerFirstName = snapshotValue["Firstname"] as! String
self.customerLastName = snapshotValue["Lastname"] as! String
self.customerAllergy = snapshotValue["Allergy"] as! String
self.customerID = snapshot.key
self.allergy.text = self.customerAllergy
self.ptname.text = "\(self.customerFirstName) \(self.customerLastName)"
}
if !taken {
print ("oops")
self.performSegue(withIdentifier: "MainPage", sender: self)
}
})
}

Answer 1

There are a number of issues with this code, and possibly your structure so let me see if I can point you in the right direction.
First, you can eliminate the taken varable as it's unneeded. In short
if snapshot.exists() {
//handle the snapshot
} else { //use else here as if the snapshot doesn't exist we want this code to run
print ("oops")
}
Second, ensure your structure is like this
Customers
cust_0
Seat: 1
cust_1
Seat: 2
etc
Third, this is a string of "1"
queryEqual(toValue: "\(seatNumber)")
and you want to query for an Int so make it
queryEqual(toValue: seatNumber)
which queries for an Int of 1
Forth:
When querying Firebase, closures will not execute when .childAdded doesn't find anything. You should use .value.
From the docs of .childAdded
This event is triggered once for each existing child and then again
every time a new child is added to the specified path.
So if no child nodes match the query, it will not execute.
Use this
refCustomer.queryOrdered(byChild: "Seat")
.queryEqual(toValue: seatNumber)
.observeSingleEvent(of: .value, with: { (snapshot) in
And... this is the important part, .value retrieves all nodes that match the query so you will need to iterate over those nodes to work with the child nodes. Assuming there would only ever be one match then you can do this
guard let allChildren = snapshot.children.allObjects as? [DataSnapshot] else {return}
let firstChild = allChildren.first
Fifth. While technically this is ok
let f = snapshotValue["Firstname"] as! String
You are guaranteeing that a Firstname node always exists. If that's true go with it. However, a safer, more Swifty way would be to do this
let f = snapshotValue["Firstname"] as? String ?? "No First Name"