Core data relationship lost after fetching more objects into the entities

问题

I have a core data model that looks like this.

Inside a tableview I load up all my appointments. On a UILabel inside my custom cell I set the appointments location name as follows.

NSString *info = appointment.location.label_vrij;

At first everything works Oké, but when I load more appointments into my database. All the info strings goes NULL. After some debugging I noticed that also appointment.location returns NULL.

This is how my NSFetchRequest looks like

RKManagedObjectStore *store = [[SanMaxDataModel sharedDataModel] objectStore];
    NSManagedObjectContext *context = store.mainQueueManagedObjectContext;

    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Appointment"];
    NSString *relationshipKeyPath = @"location"; // Set this to the name of the relationship on "A" that points to the "B" objects;
    NSArray *keyPaths = [NSArray arrayWithObject:relationshipKeyPath];
    [fetchRequest setRelationshipKeyPathsForPrefetching:keyPaths];
    NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateStart" ascending:YES];
    fetchRequest.sortDescriptors = @[descriptor];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:
                              @"tijd_dag = %@",strDate];
    [fetchRequest setPredicate:predicate];
    NSArray *matches = [context executeFetchRequest:fetchRequest error:nil];
    appointments = [matches mutableCopy];

So I think my relationship is just messed up ?

EDIT

I'm using Restkit for mapping my appointments into my core database. From the comments below I decided to turn off the option flag off the location attribute inside the Appointment entity. Because an appointment should always have a location.

Now when I load up the first day. Everything works oké. But when i try to load up the next day , I get the error: The operation couldn\U2019t be completed. (Cocoa error 1570.) Now when I take a look at the detailed Error

DetailedError: {
    NSLocalizedDescription = "The operation couldn\U2019t be completed. (Cocoa error 1570.)";
    NSValidationErrorKey = location;
    NSValidationErrorObject = "<Appointment: 0x864e2f0> (entity: Appointment; id: 0x9272c70 <x-coredata://9692683D-3077-4362-9253-652AC5B36444/Appointment/p9> ; data: {\n    autouur = 1;\n    breekuur = 0;\n    data1 = \"\";\n    data2 = \"\";\n    data3 = \"\";\n    data4 = \"\";\n    data5 = \"\";\n    data6 = \"\";\n    data7 = \"\";\n    data8 = \"\";\n    data9 = \"\";\n    dateStart = \"2013-10-23 09:00:00 +0000\";\n    dateStop = \"2013-10-23 09:30:00 +0000\";\n    duration = 30;\n    email = \"\";\n    entryID = 774294984959;\n    info = \"\";\n    \"is_blocked\" = 0;\n    \"is_except\" = 0;\n    \"is_free\" = 1;\n    \"is_moved\" = 0;\n    \"is_vert\" = 0;\n    locatieID = 773150;\n    location = nil;\n    multiID = nil;\n    serverEntryID = 774294984959;\n    serverLocatieID = 773150;\n    sms = \"\";\n    \"tijd_dag\" = 20131023;\n    \"tijd_uur\" = 900;\n})";
}

This is how I load up the JSON in core data

-(void)getAppointmentsForDate:(NSString *)date forUserID:(NSString *)userID{
    API *api = [API new];
    RKManagedObjectStore *store = [[SanMaxDataModel sharedDataModel] objectStore];
    NSLog(@"store is %@",store);
    NSManagedObjectContext *context = store.mainQueueManagedObjectContext;
    RKObjectManager *objectManager = [api mapAppointments];

    NSString *urlString = [NSString stringWithFormat:@"/doctor/1.0/json/nl/appointments/get-by-date/apikey/%@?uid=%@&date=%@",APIKey,userID,date];
   // NSString *urlString = [NSString stringWithFormat:@"/doctor/1.0/json/nl/appointments/get-by-date/apikey/%@?uid=77382&date=%@",APIKey,date];


    NSURLRequest *request = [objectManager requestWithObject:nil method:RKRequestMethodGET path:urlString parameters:nil];
    RKManagedObjectRequestOperation *operation = [objectManager managedObjectRequestOperationWithRequest:request managedObjectContext:context success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
        NSLog(@"TILL HERE IN METHOD");
        NSError *error = nil;
        [[NSNotificationCenter defaultCenter] postNotificationName:@"appointmentsLoaded" object:self];
      [[SanMaxDataModel sharedDataModel] saveToPersistentStoreAsync:&error];
     } failure:^(RKObjectRequestOperation *operation, NSError *error) {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                        message:[error localizedDescription]
                                                       delegate:nil
                                              cancelButtonTitle:@"OK"
                                              otherButtonTitles:nil];
        [alert show];
        NSLog(@"Hit error: %@", error);

    }];
    [objectManager enqueueObjectRequestOperation:operation];
}

回答1:

It looks like your problem is that by default RestKit replaces the contents of relationships when a new mapping is performed. You need to tell it that you want the new relationship data with the old. Do this by setting:

relationshipMapping.assignmentPolicy = RKAssignmentPolicyUnion;

If you're using an older version of RestKit you will need to use RKUnionAssignmentPolicy.

Where relationshipMapping is your RKRelationshipMapping instance.

来源：https://stackoverflow.com/questions/19374504/core-data-relationship-lost-after-fetching-more-objects-into-the-entities