ios NSFetchRequest，对子对象进行排序

   |  Parent         |        | Child        |
   |     - name      |------->|   - name     |
   |     - position  |        |   - position |

parent 1
   child 1
   child 2
   child 3

parent 2
   child 15
   child 16

parent 3
   child 22
   child 23
   child 24

    NSFetchRequest* fetchReqest = [[NSFetchRequest alloc] init];

    NSEntityDescription* entity = [NSEntityDescription entityForName:@\"parent\"  inManagedObjectContext:managedObjectContext];
    [fetchReqest setEntity:entity];  

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]
                                        initWithKey:@\"position\" ascending:YES];
    [fetchReqest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
    [sortDescriptor release];

    NSArray* parentsThatContainChildren =  [managedObjectContext executeFetchRequest:fetchReqest error:nil];