37.CoreData按时间查询最新20条数据

-(void)loadCertainMessages:(NSUInteger)limit
{
    NSFetchRequest *messageFetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Pre_Message"];
    messageFetchRequest.predicate = [NSPredicate predicateWithFormat:@"myAccount == %@ and toAccount == %@", [[NSUserDefaults standardUserDefaults] objectForKey:@"userPhone"],self.model.account];
    NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"time" ascending:NO];
    messageFetchRequest.sortDescriptors = [NSArray arrayWithObject:sort];
    messageFetchRequest.fetchLimit = limit;
    NSArray *resultArray = [self.managedObjectContext executeFetchRequest:messageFetchRequest error:nil];
    self.dataArray = [[[resultArray reverseObjectEnumerator] allObjects] mutableCopy];
    [self insertDateModel:self.dataArray];
    [self.cv.chatTableView reloadData];

}