How to set primary key in Swift for Realm model

For Swift 5:

import RealmSwift

     class Signature: Object {

           @objc dynamic var id = ""

            override static func primaryKey() -> String? {
                return "id"
            }
      }

To avoid: Terminating app due to uncaught exception 'RLMException', reason: 'Primary key property 'id' does not exist on object.

The return type of primaryKey() is optional:

class Foo: RLMObject {
    dynamic var id = 0
    dynamic var title = ""

    override class func primaryKey() -> String? {
        return "id"
    }
}

As of Realm Swift v10.10.0, you declare a primary key with @Persisted(primaryKey: true):

class Foo: Object {
    @Persisted(primaryKey: true) var id = 0
    @Persisted var title = ""
}

---

Older versions:

primaryKey needs to be a class function which returns the name of the property which is the primary key, not an instance method which returns the value of the primary key.

@objcMembers class Foo: RLMObject {
    dynamic var id = 0
    dynamic var title = ""

    override class func primaryKey() -> String? {
        return "id"
    }
}