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

Question

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];
to
[destinationSet addObjectsFromArray:[destinationObject allObjects]]
But I was wondering whether there is a better way to do it?
Cheers,

Answer 1

Thanks for the support from Reskit. It is now supported by RKRelationshipMapping.(https://github.com/RestKit/RestKit/issues/989)
Just set assignmentPolicy value of RKRelationshipMapping instance to RKUnionAssignmentPolicy