Swift diff realm.io without fetching it in advance

Question

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?

Answer 1

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.