CoreData and the Foreign Key database server

I have a problem concerning the CoreData and the synchronisation with a server database. When I synchronize two entities with two table on the server, the problem is that if there is a foreign key on the server, how can I manage it ? For example :

//CUSTOMERS SYNCHRONIZATION

    NSLog(@"Value : %@", [[jsonDict valueForKey:@"Customers"] objectAtIndex:0]);
    for (id object in [[jsonDict valueForKey:@"Customers"] objectAtIndex:0])
    {
        NSLog(@"%@", object);
        // if jsonDict.customers.id_customer pas présent dans table Customers
        //Recherche id_customer dans CoreData
        NSError *error = nil;

        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"id_customer = %@", [object valueForKey:@"id_customer"]]; //create a filter
        NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Customers"]; //create a query
        request.predicate = predicate; //set the filter on the query
        NSArray *customerSearch = [[self managedObjectContext] executeFetchRequest:request error:&error]; //execute the query

        if ([customerSearch count] == 0)
        {
            NSManagedObject *customers = [NSEntityDescription
                                          insertNewObjectForEntityForName:@"Customers"
                                          inManagedObjectContext:context];
            [customers setValue:[NSNumber numberWithInt:[[object valueForKey:@"id_customer"] intValue]] forKey:@"id_customer"];
            [customers setValue:[object valueForKey:@"address"] forKey:@"address"];
            [customers setValue:[object valueForKey:@"firstname"] forKey:@"firstname"];
            [customers setValue:[object valueForKey:@"lastname"] forKey:@"lastname"];
        [badges setValue:[NSNumber numberWithInt:[[object valueForKey:@"id_right"] intValue]] forKey:@"id_right"];
        }
        else if ([customerSearch count] == 1)
        {
            //Update the object
            Customers *customers = [[context executeFetchRequest:request error:&error] lastObject];

            customers.address = [object valueForKey:@"address"];
            customers.firstname = [object valueForKey:@"firstname"];
            customers.lastname = [object valueForKey:@"lastname"];
            ***customers.id_right = [NSNumber numberWithInt:[[object valueForKey:@"id_right"] intValue]];***
        }
        else
        {
            NSLog(@"WARNING - May be problems with duplicate id_customer in database"); //if > 1 then problem
        }

        if (![context save:&error])
        {
            NSLog(@"Whoops, couldn't save Customers: %@", [error localizedDescription]);
        }
    }

//RIGHTS SYNCHRONIZATION

NSLog(@"Value : %@", [[jsonDict valueForKey:@"Rights"] objectAtIndex:0]);
for (id object in [[jsonDict valueForKey:@"Rights"] objectAtIndex:0])
{
    NSLog(@"%@", object);
    // if jsonDict.customers.id_customer pas présent dans table Customers
    //Recherche id_customer dans CoreData
    NSError *error = nil;

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"id_right = %@", [object valueForKey:@"id_right"]]; //create a filter
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Rights"]; //create a query
    request.predicate = predicate; //set the filter on the query
    NSArray *customerSearch = [[self managedObjectContext] executeFetchRequest:request error:&error]; //execute the query

    if ([customerSearch count] == 0)
    {
        NSManagedObject *rights = [NSEntityDescription
                                      insertNewObjectForEntityForName:@"Rights"
                                      inManagedObjectContext:context];
        [rights setValue:[NSNumber numberWithInt:[[object valueForKey:@"id_right"] intValue]] forKey:@"id_right"];
        [rights setValue:[object valueForKey:@"name"] forKey:@"name"];
    }
    else if ([customerSearch count] == 1)
    {
        //Update the object
        Rights *rights = [[context executeFetchRequest:request error:&error] lastObject];

        rights.name = [object valueForKey:@"name"];
    }
    else
    {
        NSLog(@"WARNING - May be problems with duplicate id_customer in database"); //if > 1 then problem
    }
    if (![context save:&error])
    {
        NSLog(@"Whoops, couldn't save Rights : %@", [error localizedDescription]);
    }
}

If the answer is that I have to manage that by writing the fact that it is a relationship again on the iPad, I am sorry to say that CoreData seems to give more work than the classic management with SQLite with SQL.