Danilo Danilo - 1 year ago 48
Swift Question

Swift diff realm.io without fetching it in advance

I was wondering if there is a possibility in realm.io (swift) to select all items from one "table" that are not in the other one.

Lets say you have 2 classes:

class A: Object {
dynamic var id: Int = 0
dynamic var text: String = ""

class B: Object {
dynamic var id: Int = 0
dynamic var value: Bool = false

Is it possible to get an result of items from A who's id is not present in B?


There is actually a very simple way to do this using NSPredicate on Realm filter API.

func fetch() throws -> [A] {
        do {
            // Create Realm
            let realm = try Realm()

            // Get B objects from Realm and put their IDs to [Int] array
            let IdB: [Int] = realm.objects(B).map { $0.id }

            // Create predicate
            // Filter all items where property id is not present in array IdB
            let predicateFilter = NSPredicate(format: "NOT (id IN %@)", IdB)

            // Get all A objects from array using predicateFilter
            let objectsA = realm.objects(A).filter(predicateFilter)

            // Return the [A] array
            return objectsA.map { $0 }
        } catch {

            // Throw an error if any
            throw error


Also note that all objects from fetched using Realm are lazy loaded which means that this method is also very fast. From the documentation:

All queries (including queries and property access) are lazy in Realm. Data is only read when the properties are accessed.