How can I generate ids for objects in RealmSwift? - swift

Question

I am using RealmSwift. What is the best / canonical way to generate ids for objects?
Here is what I came up with:
class MyObject: Object {
dynamic var id = ""
dynamic var createdAt = NSDate()
override class func primaryKey() -> String {
return "id"
}
func save() {
let realm = try! Realm()
if(self.id == "") {
while(true) {
let newId = NSUUID().UUIDString
let saying = realm.objectForPrimaryKey(MyObject.self, key: newId)
if(saying == nil) {
self.id = newId
break
}
}
}
try! realm.write {
realm.add(self)
}
}
}
I wanted a function that persists the object to Realm and either overwrites or creates a new one based on the id. This seems to work ok, but I wasn't sure if there was something built into realm to do this. Or is there a better way?
Thanks.

Answer 1

I know this is a few months old, but this is how I implement auto incrementing Primary Keys.
This code is untested, but you'll get the general idea
class MyObject: Object {
/**
Primary Key
Since you can't change the primary key after the object is saved,
we'll use 0 to signify an unsaved object. When we set the primary key,
we'll never use 0.
*/
dynamic var id: Int = 0
/**
Some persisted value
*/
dynamic var someString: String?
var nextID: Int {
do {
let realm = try Realm()
/// I would use .max() but Realm only supports String and signed Int for primary keys
/// so using overflow protection, the `next` primary key would be Int.min if the
/// current value is Int.max
var id = realm.objects(MyObject.self).sorted("id", ascending: true).last?.id ?? 0
/// add 1 to id until we find one that's not in use... skip 0
repeat {
id = Int.addWithOverflow(id, 1).0 /// Add with overflow in case we hit Int.max
} while id == 0 || realm.objectForPrimaryKey(MyObject.self, key: id) != nil
return id
} catch let error as NSError {
/// Oops
fatal(error: error.localizedDescription)
}
}
convenience init(someString: String?) {
self.init()
id = nextID
self.someString = someString
save()
}
override class func primaryKey() -> String? {
return "id"
}
func save() {
/// Gotta check this in case the object was created without using the convenience init
if id == 0 { id = nextID }
do {
let realm = try Realm()
try realm.write {
/// Add... or update if already exists
realm.add(self, update: true)
}
} catch let error as NSError {
fatalError(error.localizedDescription)
}
}
}
The process for creating a unique string ID (IE: a UUID) is very similar:
class MyObject: Object {
/**
Primary Key
*/
dynamic var id: String = ""
/**
Some persisted value
*/
dynamic var someString: String?
var nextID: String {
do {
let realm = try Realm()
var id: String = NSUUID().UUIDString
/// NSUUID().UUIDString almost always makes a unique ID on the first try
/// but we'll check after we generate the first one just to be sure
while realm.objectForPrimaryKey(MyObject.self, key: id) != nil {
id = NSUUID().UUIDString
}
return id
} catch let error as NSError {
/// Oops
fatal(error: error.localizedDescription)
}
}
convenience init(someString: String?) {
self.init()
id = nextID
self.someString = someString
save()
}
override class func primaryKey() -> String? {
return "id"
}
func save() {
/// Gotta check this in case the object was created without using the convenience init
if id == "" { id = nextID }
do {
let realm = try Realm()
try realm.write {
/// Add... or update if already exists
realm.add(self, update: true)
}
} catch let error as NSError {
fatalError(error.localizedDescription)
}
}
}

Answer 2

Realm(Swift) does not currently support auto-incrementing primary keys. You can set a primary like you are above, but for auto-incrementing and unique keys, there are a couple routes that you can go:
UUID (like you have above)
Query the max "id" (primary key) of your object and set the object to be inserted as id + 1. Something like...
let id = realm.objects(MyObject).max("id") + 1
Create your own hash signature (one potential example: SHA256(epoch timestamp + SHA256(object.values))