Core Data- Using Multiple Merge Policies - core-data

Question

I am importing a large amount of data and persisting it via CoreData, setting constraints to ensure no data is duplicated (e.g. if it is imported multiple times). I do this via batch insert and use the merge policy NSMergeByPropertyStoreTrumpMergePolicy because any existing data in the store should not be overwritten upon import.
Later, the user can trigger some actions that require another batch insert, but with the merge policy NSMergeByPropertyObjectTrumpMergePolicy because now I want any changed attributes to have the new values persisted to the store.
This is all just as I need, except... based on my experimentation, there is no way to effectively CHANGE the merge policy once you have set it.
Once we make an insert using NSMergeByPropertyStoreTrumpMergePolicy, even if we now update context.mergePolicy to NSMergeByPropertyObjectTrumpMergePolicy, this will have no effect. Core Data will continue to generate SQL for the prior merge policy instead. Restarting the app and running the two steps in the opposite order verifies the same behavior -- whichever merge policy you specify first will become "stuck" in Core Data's behavior, even if you later update context.mergePolicy again in your code.
In fact, I found that even if I set two different merge policies on two different contexts (e.g. viewContext and backgroundContext) -- the behavior still gets "stuck". Whichever merge policy you use first will become the merge policy Core Data is using to generate SQL requests, even for the other context!
Can anybody verify if they have run into this and thoughts on workarounds?
I have found one thing -- which is to go all the way up the stack and create a second PersistenceController (and with it, NSPersistentContainer), using one for each merge policy. This works, but seems like overkill / not as intended by Apple?
Here is a simple SwiftUI-based project for testing.
You just need a CoreData model with an entity named Item, and two String attributes: col1 and col2. Set col1 as a constraint on Item.
And I recommend adding the launch argument to your scheme -com.apple.CoreData.SQLDebug 1 so you can see the SQL that Core Data is generating. More info on that here: https://useyourloaf.com/blog/debugging-core-data/
import SwiftUI
import CoreData
class Model: ObservableObject {
var dataArray = [[String:Any]]()
}
struct ContentView: View {
#State var model: Model = Model()
var body: some View {
VStack {
Button("Insert Data - PropertyStoreTrump") {
insertData()
}
Button("Insert More Data - PropertyObjectTrump") {
insertMoreData()
}
Spacer()
Button("Fetch Data") {
fetchAllData()
}
Button("Clear Data") {
clearAllData()
}
}
}
// Insert data using NSMergeByPropertyStoreTrumpMergePolicy
private func insertData() {
model.dataArray = [[String:Any]]()
model.dataArray.append(["col1":"1", "col2":"1"])
model.dataArray.append(["col1":"2", "col2":"1"])
model.dataArray.append(["col1":"3", "col2":"1"])
batchInsert(context: PersistenceController.shared.container.viewContext, mergePolicy: NSMergeByPropertyStoreTrumpMergePolicy)
}
// Insert data using NSMergeByPropertyObjectTrumpMergePolicy
private func insertMoreData() {
model.dataArray = [[String:Any]]()
model.dataArray.append(["col1":"2", "col2":"0"])
model.dataArray.append(["col1":"3", "col2":"0"])
model.dataArray.append(["col1":"4", "col2":"0"])
batchInsert(context: PersistenceController.shared.container.newBackgroundContext(), mergePolicy: NSMergeByPropertyObjectTrumpMergePolicy)
}
private func batchInsert(context: NSManagedObjectContext, mergePolicy: AnyObject) {
context.mergePolicy = mergePolicy
context.perform {
let insertRequest = NSBatchInsertRequest(entity: Item.entity(), objects: model.dataArray)
do {
let result = try context.execute(insertRequest)
} catch {
print("batch insert error: \(error)")
}
}
}
private func fetchAllData() {
let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Item")
do {
let results = try PersistenceController.shared.container.viewContext.fetch(fetchRequest) as! [Item]
for result in results {
print(" \(result.col1 ?? "") \(result.col2 ?? "")")
}
} catch {
print("error with fetch: \(error)")
}
}
private func clearAllData() {
let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Item")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try PersistenceController.shared.container.viewContext.execute(batchDeleteRequest)
} catch {
print("error with delete request: \(error)")
}
}
}
struct PersistenceController {
static let shared = PersistenceController()
let container: NSPersistentContainer
init(inMemory: Bool = false) {
container = NSPersistentContainer(name: "CoreDataStackOverflow")
if inMemory {
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
container.viewContext.automaticallyMergesChangesFromParent = true
}
}
The workaround I found is to add this line to PersistenceController
static let shared2 = PersistenceController()
then use PersistenceController.shared.container... for one merge policy
and PersistenceController.shared2.container... for the other