NSBatchDeleteRequest causes Merge Conflict

Question

I have an application that will sync with a server with data that can change daily. During the sync, I remove all the data for some entities and reload it with new data. I am using the following code:

    func SyncronizeUserComments(theData : [[AnyHashable : Any]])
{
    // Delete User Comments for this User and Connection
    let commentRequest : NSFetchRequest<NSFetchRequestResult> = PT_UserComments.fetchRequest()
    commentRequest.predicate = NSPredicate(format: "connection = %@ AND user == %@", Global_CurrentConnection!, Global_CurrentUser!)
    coreData.processDeleteRequest(request: commentRequest)

    // ADD the Comments to CoreData
    for index in 0..<theData.count {
        let result : [AnyHashable : Any] = theData[index]
        if let commentID = result["Comment_ID"] as? String, let commentText = result["Comment_Text"] as? String, let commentTitle = result["Comment_Title"] as? String
        {
            let newUserComment = PT_UserComments(context: coreData.persistentContainer.viewContext)
            newUserComment.connection = Global_CurrentConnection
            newUserComment.user = Global_CurrentUser
            newUserComment.comment_ID = commentID
            newUserComment.comment_Text = commentText
            newUserComment.comment_Title = commentTitle
        }
    }

    // Add the User Comments
    print("Added New User Comments: \(theData.count)")
    coreData.saveContext()
}

    func processDeleteRequest(request : NSFetchRequest<NSFetchRequestResult>)
{
    let deleteRequest = NSBatchDeleteRequest(fetchRequest: request)
    deleteRequest.resultType = .resultTypeObjectIDs

    do {
        let result = try coreData.persistentContainer.viewContext.execute(deleteRequest) as? NSBatchDeleteResult
        let objectIDArray = result?.result as? [NSManagedObjectID]
        let changes = [NSDeletedObjectsKey : objectIDArray]
        NSManagedObjectContext.mergeChanges(fromRemoteContextSave: changes as Any as! [AnyHashable : Any], into: [coreData.persistentContainer.viewContext])
    } catch  {
        fatalError("Fatal Error Deleting Data: \(error)")
    }

    coreData.saveContext()
}

When I call coreData.saveContext() I will get a Merge Conflict against the deleted data.

In reading about CoreData and the NSBatchDeleteRequest, this deletes at the SQL LITE level and bypasses the in memory cache.

The only way I have been able to get this to work is by setting:

context.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy

Is this correct, or am I doing something wrong? I am also setting this merge policy in my saveContext() in the Core Data Stack.

Answer 1

I just spent hours debugging the same issue, hopefully this can help someone.

The problem is that NSManagedObjectContext.mergeChanges(fromRemoteContextSave:, into:) updates the managed object context but does not update the row cache version number of the deleted objects relationships to match the updated version number (Z_OPT) in the database file, causing a mismatch at time of the save.

If you're using NSErrorMergePolicyType this will cause the next save to fail, (or even a later one when the relationships become flagged for save), even though everything but the version numbers match. I've not seen this mentioned in the related docs or WWDC video, but I guess Apple assumed people would always pick a non-default merge policy.

So picking NSMergeByPropertyStoreTrumpMergePolicy solves it, as mentioned in the question, but you might not want this policy for all your save operations. To avoid that I ended up writing a custom merge policy that only resolves version mismatches. The code is below (this is untested Swift as I originally wrote in Obj-C, but should be equivalent):

//Configure the merge as below before saving
context.mergePolicy = AllowVersionMismatchMergePolicy(merge: .errorMergePolicyType)

//...

//The custom merge policy
class AllowVersionMismatchMergePolicy: NSMergePolicy {
    override func resolve(optimisticLockingConflicts list: [NSMergeConflict]) throws {
        do {
            //if the default resolve worked leave it alone
            return try super.resolve(optimisticLockingConflicts: list)
        } catch {
            //if any of the conflict is not a simple version mismatch (all other keys being equal), fail
            let hasValueConflict = list.contains { conflict -> Bool in
                //compare object and row cache
                if let objectSnapshot = conflict.objectSnapshot as NSObject?,
                    let cachedSnapshot = conflict.cachedSnapshot as NSObject? {
                    return !objectSnapshot.isEqual(cachedSnapshot)
                }
                //compare row cache and database
                if let cachedSnapshot = conflict.cachedSnapshot as NSObject?,
                    let persistedSnapshot = conflict.persistedSnapshot as NSObject? {
                    return !cachedSnapshot.isEqual(persistedSnapshot)
                }
                //never happens, see NSMergePolicy.h
                return true
            }
            if hasValueConflict {
                throw error
            }

            //Use store rollback merge policy to resolve all the version mismatches
            return try NSMergePolicy.rollback.resolve(optimisticLockingConflicts: list)
        }
    }
}