Question

这是如何创建 NSManagedObject 的

NSEntityDescription *entity = [NSEntityDescription entityForName:strEntityName inManagedObjectContext:managedObjContext];
NSManagedObject * managedObject = (NSManagedObject *)[[NSClassFromString(strEntityName) alloc] initWithEntity:entity insertIntoManagedObjectContext:managedObjContext];
//values are mapped into this object

现在将 NSManagedObject 保存到持久存储并获取当前插入的对象，如下所示：

NSError *error;
BOOL isDone = [managedObjectContext save:&error];
//BOOL isDone = [managedObjectContext obtainPermanentIDsForObjects:[NSArray arrayWithObjects:tempManagedObject, nil] error:&error];
if (isDone && error == nil){

   //fetch last inserted object here

   //make fetch request
   NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:strEntityName];
   //make query using fetch request in context      
   NSError *error;
   NSArray *arrFetchRequest = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
   if (arrFetchRequest.count>0){
       //This is last inserted object
       NSManagedObject *managedObject = [arrFetchRequest lastObject];
       return managedObject;
   }
}

我也提到了Swift - How to get last insert id in Core Data, store it in NSUserDefault and predicate。但它不会在NSManagedObject中拥有永久对象ID，因为我们正在保存临时ID并使用它来获取，而我们根本没有永久ID。

Canyone还有其他选择吗？

Answer 1

在上下文中插入NSManagedObject时，它将具有temporaryID（这意味着它尚未保存到数据库中）。

因此需要在上下文中为NSManagedObject获取permanentID（这意味着它已保存到数据库），使用obtainPermanentIDsForObjects并且相同的NSManagedObject将具有permanentID

if (insertManagedObject)
{
    NSLog(@"Before %@ : %d",insertManagedObject.objectID,insertManagedObject.objectID.isTemporaryID);
    //Obtain permanentID for object
    NSError *error;
    BOOL hasObtainedPermanentID = [managedObjectContext obtainPermanentIDsForObjects:[NSArray arrayWithObjects:insertManagedObject, nil] error:&error]; //;
    //check if its done
    if (hasObtainedPermanentID && error == nil){

       NSLog(@"After %@ : %d",insertManagedObject.objectID,insertManagedObject.objectID.isTemporaryID);

       //check context has changes and is saved in context
       if ([managedObjectContext hasChanges] && [managedObjectContext save:&error])
       {
            return insertManagedObject;
       }

    }

}

最后从CoreData插入NSManagedObject

1 个答案: