核心數據：問題插入數據與關係

Question

我有兩個實體（類別，事件），他們有雙向多對多關係。一個類別可以有多個事件，一個事件可以有多個類別。類別到事件的關係是可選的，因爲類別可以在沒有事件的情況下存在，但事件到類別的關係是強制性的（事件不能在沒有類別的情況下存在）。我試圖插入事件並添加類別給他們，但我得到NSValidationErrorValue =關係錯誤。這是我的代碼：

private func storeEventsXMLStream(_ xml: XMLIndexer) { 

    let managedObjectContext = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType) 
    managedObjectContext.persistentStoreCoordinator = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.persistentStoreCoordinator 

    // Remove all data before inserting 
    // This line of code is necessary because data needs to be downloaded on daily basis. 
    // Otherwise, I will get redundant data. 
    removeAllExistingData("Event_Ottawa", managedObjectContext: managedObjectContext) 

    autoreleasepool { // Scoping is necessary to fix memory leak 
     for xmlcat in xml["events"]["event"]{ 
      let event = NSEntityDescription.insertNewObject(forEntityName: "Event_Ottawa", into: managedObjectContext) as! Event_Ottawa 
      event.id = Int32((xmlcat.element?.attribute(by: "id")?.text)!)! 
      event.website_url_english = xmlcat["website_url_english"].element?.text 
      event.website_url_french = xmlcat["website_url_french"].element?.text 
      // setting other attributes of events here. Exactly like I did in above 3 line 

      // Just another attribute. Storing it a String in Coredata 
      var recur_rules = "" 
      for rule in xmlcat["recur_rules"]["recur_rule"] { 
       recur_rules += (rule.element?.attribute(by: "weekday")?.text)! 
      } 
      if !recur_rules.isEmpty { 
       event.recur_rules = recur_rules 
      } 


      do { 

       var predicateArray:[NSPredicate] = [] 

       // Categories are inserted to the Coredata before this method call. So I'm fetching the applicable one here. 
       for category in xmlcat["categories"]["category"] { 
        let predicate = NSPredicate(format: "id = %@", (category.element?.attribute(by: "id")?.text)!) 
        predicateArray.append(predicate) 
       } 

       let requestCategory:NSFetchRequest<Category_Event_Ottawa> = Category_Event_Ottawa.fetchRequest() 
       requestCategory.predicate = NSCompoundPredicate.init(andPredicateWithSubpredicates: predicateArray) 
       let managedContext = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType) 
       managedContext.persistentStoreCoordinator = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.persistentStoreCoordinator 

       let applicableCategories = try managedContext.fetch(requestCategory) 

       for category in applicableCategories { 
        event.addToCategory(category) 
       } 


       } 
      } catch { 
       print(error) 
      } 

     } 

    } 

    // only save once per batch insert 
    do { 
     try managedObjectContext.save() 
    } catch { 
     print(error) 
    } 

    managedObjectContext.reset() <-- I get EXC_BAD_ACCESS, when I use the same object context for fetching as insertion 

} 

我收到的錯誤：

Error Domain=NSCocoaErrorDomain Code=1560 "(null)" UserInfo={NSDetailedErrors=(
"Error Domain=NSCocoaErrorDomain Code=1550 \"The operation couldn\U2019t be completed. (Cocoa error 1550.)\" UserInfo={Dangling reference to an invalid object.=null, NSValidationErrorValue=Relationship 'category' on managed object (0x6100050963f0) 

如果使用插入和取出相同的管理對象方面，我沒有得到這個錯誤了。但是我從重置上下文對象的行中獲取EXC_BAD_ACCESS。我從另一篇文章中瞭解到CoreData不是線程安全的。所以也許這是一個問題。但我該如何解決這個問題？如果相關，則事件類別關係的刪除規則爲Nullify，而類別事件爲級聯。

Answer 1

在這一行：

requestCategory.predicate = NSCompoundPredicate.init(andPredicateWithSubpredicates: predicateArray) 

你創建與AND將一個複合謂語。你要求核心數據獲取你所有的id = 7和id = 8和...等類別。這是行不通的。該類別只能有一個ID。在這種情況下，你實際上需要一個'或'謂詞。

但是，我認爲更好的方法是在您開始循環XML之前將所有類別加載到由其ID標識的字典中，然後將它們從字典中拉出。這比每次讀取都要高效得多。

此外，您不能在單獨的上下文中獲取類別，然後在單獨的上下文中創建對象之間的關係。核心數據將在您嘗試時崩潰。