Question

在我的应用程序中我在core data中有一些记录，我想按升序获取最后20条记录，即如果有30条记录，那么我想按升序从11到30获取记录 -

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Messages" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    [fetchRequest setFetchLimit:20];
    NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                              initWithKey:@"messageId" ascending:YES];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"((userId == %@) && (frndId == %@) )",uid, fid];
    [fetchRequest setPredicate:predicate];

    NSError *error;
    NSArray* dataArray = [[managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];
    for (Messages* msg in dataArray) {
//code here
}

但它按升序给我前20条记录，如果我改为

NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                                  initWithKey:@"messageId" ascending:NO];

然后我得到了最后20条记录，但这是按升序排列的？任何建议都会受到赞赏。

Answer 1

你不能设置这个偏移量吗？

//assume it fits an nsinteger
NSInteger count = [managedObjectContext countForFetchRequest:fetchRequest /*the one you have above but without limit */ error:&error];

NSUInteger size = 20;
count -= size;
[fetchRequest setFetchOffset:count>0?count:0];
[fetchRequest setFetchLimit:size];
NSArray* dataArray = [managedObjectContext executeFetchRequest:fetchRequest error:&error]

Answer 2

NSFetchRequest没有为此提供方法。您必须按降序获取20条记录，然后反转生成的数据数组：

dataArray = [[dataArray reverseObjectEnumerator] allObjects];

如果这是用于在表视图中显示，那么而不是实际反转数据数组您可以调整cellForRowAtIndexPath方法来显示对象

dataArray[dataArray.count - 1 - indexPath.row]

而不是

dataArray[indexPath.row]

Answer 3

你可以使用 -

//your code as it is till this line but with 
//NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                              initWithKey:@"messageId" ascending:NO];
NSArray* dataArray = [[managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];

    NSArray *sortedArray;
    sortedArray = [dataArray sortedArrayUsingComparator:^NSComparisonResult(id a, id b) {
        NSString *first = [(Messages*)a messageId];
        NSString *second = [(Messages*)b messageId];
        return [first compare:second];
    }];
    for (Messages* msg in sortedArray) {
//your code
}

快乐编码：P

核心数据按升序获取最后20条记录？

3 个答案: