Realm - Can't create object with existing primary key value

I have a object Person with many dogs. App has separate page where it shows just dogs and other page where it shows person's dogs

My model is as follows

class Person: Object {
    dynamic var id = 0
    let dogs= List<Dog>()

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

class Dog: Object {
    dynamic var id = 0
    dynamic var name = ""

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

I have persons stored in Realm. Person has detail page where we fetch and show his dogs. If dog already exist, I update latest info for that dog and add it to person's dog list else create new dog, save it and add it to persons list. This works in coredata.

// Fetch and parse dogs
if let person = realm.objects(Person.self).filter("id =\(personID)").first {
    for (_, dict): (String, JSON) in response {
        // Create dog using the dict info,my custom init method
        if let dog = Dog(dict: dict) {
            try! realm.write {
                // save it to realm
                realm.create(Dog, value:dog, update: true)
                // append dog to person
                person.dogs.append(dog)
            }
        }
    }
    try! realm.write {
        // save person
        realm.create(Person.self, value: person, update: true)
    }
}

On trying to update person with his dogs,realm throws exception Can't create object with existing primary key value

The problem here is that even though you're creating a completely new Realm Dog object, you're not actually persisting that one to the database, and so when you call append, you're trying to add a second copy.

When you call realm.create(Dog.self, value:dog, update: true), if an object with that ID already exists in the database, you're simply updating that existing object in with the values in the dog instance you created, but that dog instance is still an independent copy; it's not the Dog object in the database. You can confirm this by checking if dog.realm is equal to nil or not.

So when you call person.dogs.append(dog), because dog is not already in the database, Realm tries to create a whole new database entry, but fails because there is already a dog with that ID.

If you want to append that dog object to a person, it'll be necessary to query Realm to retrieve a proper dog object that's referencing the entry in the database. Thankfully this is really easy with Realm objects backed by primary keys since you can use the Realm.object(ofType:forPrimaryKey:) method:

if let person = realm.object(ofType: Person.self, forPrimaryKey: "id") {
    for (_, dict): (String, JSON) in response {
        //Create dog using the dict info,my custom init method
        if let dog = Dog(dict: dict)
        {
            try! realm.write {
                //save it to realm
                realm.create(Dog.self, value: dog, update: true)
                //get the dog reference from the database
                let realmDog = realm.object(ofType: Dog.self, forPrimaryKey: "id")
                //append dog to person
                person.dogs.append(realmDog)
            }
        }
    }
    try! realm.write {
        //save person
        realm.create(person .self, value: collection, update: true)
    }
}