NSBatchDeleteRequest导致合并冲突

Question

我有一个应用程序，它将与服务器同步，并每天更新数据。在同步期间，我删除了某些实体的所有数据，并重新加载了新数据。我正在使用以下代码：

    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()
}

当我调用coreData.saveContext（）时，我将对已删除的数据产生合并冲突。

在阅读有关CoreData和NSBatchDeleteRequest的内容时，这会在SQL LITE级别删除，并绕过内存缓存。

我能够使它起作用的唯一方法是设置：

context.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy

这是正确的，还是我做错了什么？我也在Core Data Stack的saveContext（）中设置此合并策略。

Answer 1

我刚刚花了几个小时调试同样的问题，希望这可以帮助别人。

问题是NSManagedObjectContext.mergeChanges(fromRemoteContextSave:, into:)更新托管对象上下文，但不更新已删除对象关系的行缓存版本号以匹配Z_OPT数据库文件中更新的版本号 ( )，从而导致保存时不匹配。

如果您使用NSErrorMergePolicyType此选项，即使除版本号之外的所有内容都匹配，也会导致下一次保存失败（甚至当关系标记为保存时，稍后的保存也会失败）。我没有在相关文档或 WWDC 视频中看到这一点，但我猜苹果认为人们总是会选择非默认合并策略。

因此NSMergeByPropertyStoreTrumpMergePolicy，正如问题中提到的，选择解决了这个问题，但您可能不希望对所有保存操作使用此策略。为了避免这种情况，我最终编写了一个仅解决版本不匹配问题的自定义合并策略。代码如下（这是未经测试的 Swift，正如我最初在 Obj-C 中编写的，但应该是等效的）：

//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)
        }
    }
}