ios - NSBatchDeleteRequest 导致合并冲突

Question

我有一个应用程序，它将与服务器同步，其中的数据每天都会更改。在同步期间，我删除了某些实体的所有数据并使用新数据重新加载。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，我将收到针对已删除数据的合并冲突。

在阅读有关 CoreData 和 NSBatchDeleteRequest 的内容时，它会在 SQL LITE 级别进行删除并绕过内存缓存。

我能够让它工作的唯一方法是设置:

context.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy

这是正确的还是我做错了什么？我还在核心数据堆栈的 saveContext() 中设置了此合并策略。

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。

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

如果您使用NSErrorMergePolicyType，这将导致下一次保存失败(甚至当关系被标记为保存时稍后的保存失败)，即使除了版本号之外的所有内容都匹配。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，选择 NSMergeByPropertyStoreTrumpMergePolicy 可以解决该问题，如问题中所述，但您可能不希望在所有保存操作中使用此策略。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(这是我最初在 Obj-C 中编写的未经测试的 Swift，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)
        }
    }
}