我在保存实体（DictionaryMarketSubcategoryEntity）后出现此错误，而不是再次保存它们。

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<DictionaryMarketSubcategoryEntity 0x60000269e2b0> setValue:forUndefinedKey:]: the entity DictionaryMarketSubcategoryEntity is not key value coding-compliant for the key "(null)".'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007ff8004278cb __exceptionPreprocess + 242
    1   libobjc.A.dylib                     0x00007ff80004dba3 objc_exception_throw + 48
    2   CoreFoundation                      0x00007ff80042753d -[NSException init] + 0
    3   CoreData                            0x00007ff8049b8a02 -[NSMergePolicy _mergeToManyUnionRelationshipsForObject:andObject:] + 945
    4   CoreData                            0x00007ff8049bcf29 -[NSMergePolicy resolveConstraintConflicts:error:] + 5412
    5   CoreData                            0x00007ff8049bb99a -[NSMergePolicy resolveConflicts:error:] + 210
    6   CoreData                            0x00007ff80497a534 -[NSManagedObjectContext save:] + 3215
    7   FCDev                               0x0000000101624e04 $s5FCDev13CoreDataStackC20saveContextIfChangedyyF + 1524
    8   FCDev                               0x0000000101573f8b $s5FCDev36CoreDataStorageServiceImplementationC5writeyyF + 43
    9   FCDev                               0x0000000101575710 $s5FCDev36CoreDataStorageServiceImplementationCAA0dE0A2aDP5writeyyFTW + 16
    10  FCDev                               0x0000000100a02291

这是我保存的实体（DictionaryMarketSubcategoryEntity）。它有子类别（DictionaryMarketSubcategoryEntity）。如果我注解并删除子类别，一切正常。似乎当我保存DictionaryMarketCategoryEntity数组时，它保存了子实体（DictionaryMarketSubcategoryEntity），重新保存后，合并策略崩溃。为什么？

下面是初始化代码：

extension DictionaryMarketCategoryEntity: JSONInitializable {
    convenience init(json: JSON) {
        self.init(context: AppInstances.shared.storage.context)

        self.backgroundColor = json["backgroundColor"].stringValue
        self.backgroundImagePath = json["backgroundImage"].stringValue
        self.descriptionText = json["description"].stringValue
        self.icon = json["icon"].stringValue
        self.id = json["id"].stringValue
        self.imagePath = json["image"].stringValue
        self.parent = json["parent"].stringValue
        self.span = json["span"].int16Value
        self.title = json["title"].stringValue
        self.type = json["type"].stringValue

        let childrenJson = json["children"].arrayValue
        let childrenEntities = childrenJson.map { DictionaryMarketSubcategoryEntity(json: $0) }
        let childrenSet = NSSet(array: childrenEntities)
        addToChildren(childrenSet)
    }
}

extension DictionaryMarketSubcategoryEntity {
    convenience init(json: JSON) {
        self.init(context: AppInstances.shared.storage.context)

        self.id = json["id"].stringValue
        self.title = json["title"].stringValue
        self.descriptionText = json["description"].stringValue
        self.imagePath = json["image"].stringValue
        self.backgroundImagePath = json["backgroundImage"].stringValue
        self.backgroundColor = json["backgroundColor"].stringValue
        self.icon = json["icon"].stringValue
        self.parent = json["parent"].stringValue
        self.span = json["span"].int16Value
        self.type = json["type"].stringValue
    }
}