Get Core Data Entity relatives with a generic function

Question

I'm designing a data manager for my Core Data model and I'd like to create a generic function to fetch relatives of a class.

I’ve created a protocol allowing to build managers for each data type. In this protocol I already defined two associated types T and K and several simple functions. Now I’m stuck with a class relatives fetching method — I need to indicate somehow that T has K relatives. I’ve tried in vain to create some protocol indicating this relationship thru mutual properties, so both classes could conform to this protocol. Any idea, is it even possible?

import Foundation
import CoreData

protocol DataManager {

    associatedtype T: NSManagedObject, NSFetchRequestResult
    associatedtype K: NSManagedObject, NSFetchRequestResult // Relative

    static var sharedInstance: Self { get }

    static func getAll(sorted: [NSSortDescriptor]?, context: NSManagedObjectContext) -> [T]?
    static func insert(item: T)
    static func update(item: T)
    static func clean()
    static func deleteById(id: String)

    // Relatives
    static func getRelatives(by: T) -> [K]?
    static func get(byRelative: K) -> [T]?
}

extension DataManager {

    static func getAll(sorted: [NSSortDescriptor]?, context: NSManagedObjectContext) -> [T]? {

        guard let fetchRequest: NSFetchRequest<T> = T.fetchRequest() as? NSFetchRequest<T> else { return nil }
        fetchRequest.sortDescriptors = sorted

        var results: [T]? = nil

        do {
            results = try context.fetch(fetchRequest)
        } catch {
            assert(false, error.localizedDescription)
        } //TODO: Handle Errors

        return results
    }
}


protocol Identifiable {
    typealias Identity = String
    var id: Identity? { get }
}


extension DataManager where Self.T: Identifiable {

    static func get(by id: T.Identity, context: NSManagedObjectContext) -> T? {

        guard let fetchRequest: NSFetchRequest<T> = T.fetchRequest() as? NSFetchRequest<T> else { return nil }

        fetchRequest.predicate = NSPredicate(format: "%K == %@", "id", id)

        var rawResults: [T]? = nil

        do {
            rawResults = try context.fetch(fetchRequest)
        } catch {
            assert(false, error.localizedDescription)
        } //TODO: Handle Errors


        if let result = rawResults?.first {
            return result }
        else { return nil }
    }
}

Answer 1

Well, I've created one solution. We can identify all relations with a particular class:

let relationships = T.entity().relationships(forDestination: K.entity())

It allows us to find all IDs of an item for each relationship (we can have many relationships for the same relative Entity):

let relativesIDs = item.objectIDs(forRelationshipNamed: relationship.name)

So, we can use these IDs to fetch records from another class.

static func getRelatives(of item: T, context:NSManagedObjectContext) -> [K]? {

    guard let fetchRequest: NSFetchRequest<K> = K.fetchRequest() as? NSFetchRequest<K> else { return nil }
    fetchRequest.fetchBatchSize = 100

    var results: [K]? = nil

    var resultSet: Set<K> = [] // doesn't allow duplicates

    let relationships = T.entity().relationships(forDestination: K.entity())

    for relationship in relationships {

        let relativesIDs = item.objectIDs(forRelationshipNamed: relationship.name)
        let predicate = NSPredicate(format: "self IN %@", relativesIDs)
        fetchRequest.predicate = predicate

        var batchResults: [K] = []

        do {
            batchResults = try context.fetch(fetchRequest)
        } catch {
            assert(false, error.localizedDescription)
        } //TODO: Handle Errors

        if batchResults.count > 0 { resultSet = resultSet.union(Set(batchResults)) }
    }

    if resultSet.count > 0 { results = Array(resultSet) }

    return results
}

I'm not sure that this is the most elegant solution, but it works :-)