Restkit to-many relationship append to set instead of setting a new set

问题

I have a iOS Restkit related question. I have a parent-child relationship data coming from a remote server and map those object to a NSManagedObject object with Restkit. The problem that I am currently having is every request to the server always wipe out the "child" relationship and replace it with the new data coming from the server. Is there a way to avoid those and append the new child instead?

For example: I have a classic Category --> Products relationship.

{"categories": [
    {
        "cat_id": "1",
        "cat_title": "category 1",
        "cat_tag": 1,
        "product": [
            {
                "prod_id": "1",
                "prod_name": "product 1",
                "prod_tag": 1
            },
            {
                "prod_id": "2",
                "prod_name": "product 2",
                "prod_tag": 1
            }
        ]
    }
] }

And that works fine and everything is saved properly with the relationship on the CoreData. But if I make another request to the server and have a new response:

{"categories": [
    {
        "cat_id": "1",
        "cat_title": "category 1",
        "cat_tag": 1,
        "product": [
            {
                "prod_id": "3",
                "prod_name": "product 3",
                "prod_tag": 1
            },
            {
                "prod_id": "4",
                "prod_name": "product 4",
                "prod_tag": 1
            }
        ]
    }
] }

I will have product 3 and product 4 replace product 1 and product 2 on the database. I am sure I setup all the relationship and primary key correctly. (Both cat_id and prod_id are set as a primary key).

Having investigated through the RestKit's internal framework, I noticed that around line 576 in the RKObjectMappingOperation class, there is

RKLogTrace(@"Mapped NSSet relationship object from keyPath '%@' to
           '%@'. Value: %@", relationshipMapping.sourceKeyPath,
           relationshipMapping.destinationKeyPath, destinationObject);

NSMutableSet *destinationSet = [self.destinationObject
                         mutableSetValueForKey:relationshipMapping.destinationKeyPath];
[destinationSet setSet:destinationObject];

So I guess that is easy to just change

[destinationSet setSet:destinationObject];

[destinationSet addObjectsFromArray:[destinationObject allObjects]]

But I was wondering whether there is a better way to do it?

Cheers,