ios - 如果从Feed中删除对象，则从Core Data中删除对象

Question

考虑提要中的字典对象与核心数据中的排序实体之间的以下关联:

Feed  CoreData
----  --------
A     A
B     B
C     C
D     D

在枚举提要时，我检查了实体中A的[stringForKey:@"name"]是否为EqualTo A.name。如果匹配，我将更新实体。如果不是，我将一个新实体插入CoreData。

这适用于更新和插入，但不适用于删除。考虑将对象C从提要中删除:

Feed  CoreData
----  --------
A     A
B     B
D     C
      D

当我在Feed中找到“D”时，它将看到CoreData中的对象“C”不匹配，并创建了一个新对象D。因此，我现在有两个问题:我有两个“D”对象，而对象“C”不会从CoreData中删除。

因此，虽然我想结束此:

Feed  CoreData
----  --------
A     A
B     B
D     D

我现在得到的是:

Feed  CoreData
----  --------
A     A
B     B
D     C
      D
      D

这肯定是一个常见问题，所以我想知道确定何时从Core Data中删除实体的最佳实践是什么。

Answer 1

这是我遍历项目以确定是更新，插入还是删除操作:

-(void)updateWithJSON:(id)JSON
{
    //Get an array of all related managed objects
    NSMutableArray *allContacts = [[NSMutableArray alloc] initWithArray:[self getAllContacts]];

    //Loop through each object downloaded from the server
    for (NSDictionary *objectInfo in [JSON objectForKey:@"Contacts"])
    {
        NSString *objectKey = [objectInfo objectForKey:@"BackendID"];

        //Get the managed object for the objectKey
        Contact *contact = [[allContacts filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"backendID == %@", objectKey]] lastObject];

        //If the object is nil, then insert the object
        if (contact == nil)
        {
            NSLog(@"Object with key %@ is new.", objectKey);
            contact = [[Contact alloc] initWithEntity:[NSEntityDescription entityForName:@"Contact" inManagedObjectContext:self.managedObjectContext] insertIntoManagedObjectContext:self.managedObjectContext];
            contact.backendID = objectKey;
        }

        //Assign property values
        contact.firstName = [objectInfo objectForKey:@"FirstName"];
        contact.lastName = [objectInfo objectForKey:@"LastName"];
        contact.jobTitle = [objectInfo objectForKey:@"JobTitle"];
        contact.department = [objectInfo objectForKey:@"Department"];
        contact.email = [objectInfo objectForKey:@"Email"];
        contact.fax = [objectInfo objectForKey:@"Fax"];
        contact.primaryPhone = [objectInfo objectForKey:@"PrimaryPhone"];
        contact.secondaryPhone = [objectInfo objectForKey:@"SecondaryPhone"];

        //Remove the object from the array of all the objects
        if ([allContacts containsObject:contact])
            [allContacts removeObject:contact];
    }

    //Delete any objects that still remain in the array (means they were deleted server-side
    for (Contact *contact in allContacts) {
        NSLog(@"Removing Contact with key %@", contact.backendID);
        [self.managedObjectContext deleteObject:contact];
    }

    NSError *error = nil;
    [self.managedObjectContext processPendingChanges];
    [self.managedObjectContext save:&error];

    if (error)
    NSLog(@"Error Saving Contacts: %@", error.localizedDescription);
}