Duplicate NSManagedObjects in Core Data using MagicalRecord and MMRecord - afnetworking

Question

Update 21/7/2014: I have found a workaround for the problem described here. I have created a separate NSManagedObjectContext called downloadContext. The downloaded serialized objects are put in that separate context. I then iterate through the response array and check if the object already exists in the MR_defaultContext (using the primary key).
If it does I update the properties from the downloaded object with the same primary key. It is not possible to simply copy the whole object and thus overwriting all properties of the existing object (because the updated object and the old one are in separate contexts). Instead I copy all properties of the downloaded object into an NSDictionary (dictionaryWithValuesForKeys) and then use setValuesForKeysWithDictionary to overwrite the properties of the old object. The same has to be done with the child objects.
If the object is new, I create a new object and copy the properties in the same way. I will need to check if this solution is efficient enough or if it will occupy too much memory in case there are many objects to download (first sync). The alternative is to create my own serialiser that decides on the basis of primary keys if to update an existing object or to create a new one.
For this issue, please note that I have seen the discussion on the relatedByAttribute key setting. It doesn't help in this case since this might only avoid duplicates of child relationship objects ( I haven't checked if there are duplicates). This is about the parent object duplicates.
In my project, I use AFNetworking, MMRecord and MagicalRecord together as dependencies. The situation is as follows:
I create a response serializer from the AFMMRecordResponseSerializer class which I simply set to my subclass object of the AFHTTPSessionManager. With this session manager I send a GET-request to the server. The response object already contains serialized NSManagedObjects (they are actually a subclass of MMRecord). I have set the MMRecordEntityPrimaryAttributeKey to each of the NSManagedObject's intended primary key. I think that the problem lies with MagicalRecords though, because it simply ignores MMrecord's primary attribute key setting.
After the initial GET-request with the persistent store still empty, there is no modification/creation date predicate. In case there are already persisted objects, the GET-request contains a predicate retrieving only objects after the last mod date from the server. So the updated object is persisted but the older object is not overwritten. When I display the titles of the objects in a table view I can see that the older version and updated version of an object are both displayed. Important: the table view is populated only from core data, not directly from the responseObject (containing the serialized objects).
Below is the most important code.
Thanks for any hints in the right direction
Dominik
-(void) fetchNotesFromServer
{
NSLog(#"fetchNotesFromServer called");
self.sessionManager.responseSerializer = [self getSerializerForEntityName:#"NAS_Notes" AndEndPointPathComponent:#"DBFetchNotes.php?"];
[self.sessionManager GET:[self getFetchNotesExtension] parameters:nil
success:^(NSURLSessionDataTask *task, id responseObject)
{
if (responseObject )
{
NSLog(#"fetch all notes response: %#", responseObject);
self.latestNotes = (NSMutableArray *)responseObject;
if ([self.latestNotes count] > 0)
{
[self saveDownloadedNotesInContext:[NSManagedObjectContext MR_defaultContext]];
}
}
} failure:^(NSURLSessionDataTask *task, NSError *error)
{
NSLog(#"NSerror %#", error);
}];}
-(void) saveDownloadedNotesInContext:(NSManagedObjectContext *)context{
[context MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
if (success) {
[self.delegate didFinishNotesSync:self];
}
else if (error) {
NSLog(#"error saving to persistent store %#", error);
}
}];
}