问题描述
我正在查询核心数据，以在一群人中找到最旧的"和最重的".对于较大的数据集，这通常是可行的(因为重复匹配的机会较少)，但是对于较小的数据集，其中最老的人也可能是最重的人，我遇到了问题.
I am querying Core Data to find the "OLDEST" and "HEAVIEST" amongst a group of people. With a larger dataset this usually works (as the chance of a duplicate match is less), but with a small dataset where the OLDEST person may also be the HEAVIEST I am having problems.
[John, 77, 160]
[Pete, 56, 155]
[Jane, 19, 130]
[Fred, 27, 159]
[Jill, 32, 128]
由于我想在2个 UITableView 的UITableViewCell 中显示此信息，因此我首先要运行2个 NSFetchRequests (找到最旧的，第二个找到最重的)我是他们为每个托管对象获取 objectID 并将它们添加到我用来设置我的最终 NSFetchRequest 的过程中 NSFetchedResultsController .
As I want to display this information in 2 UITableViewCells of a UITableView I am doing this by first running 2 NSFetchRequests (one to find the OLDEST, the second to find the HEAVIEST) I am them taking the objectIDs for each managed object and adding them to a final NSFetchRequest that I am using to setup my NSFetchedResultsController.
// FETCH REQUEST - OLDEST, HEAVIEST
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"People"];
NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:@"age" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:descriptor]];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF IN %@", IDArray];
[fetchRequest setPredicate:predicate];
如果我打印最终的 NSFetchRequest 的描述"，则确实包含2个指向managedObjects的指针.
If I "Print the Description" of the final NSFetchRequest it does indeed contain 2 pointers to managedObjects.
(i.e. [John, 77, 160] [John, 77, 160])
我的问题似乎是当我这样做
My problem seems to be that when I do
[[self fetchedResultsController] performFetch:nil];
UITableView 委托方法:
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
NSArray *sectionArray = [[self fetchedResultsController] sections];
id <NSFetchedResultsSectionInfo> sectionInfo = [sectionArray objectAtIndex:0];
NSUInteger numberOfRows = [sectionInfo numberOfObjects];
NSLog(@"ROWS: %u", numberOfRows);
return numberOfRows;
}
仅将 numberOfRows 显示为1，并且在我的 UITableView 中仅显示单个[John，77，160].
only shows the numberOfRows as 1 and only displays a single [John, 77, 160] in my UITableView.
推荐答案
获取请求不会返回重复的对象.它返回所有与谓词匹配的对象.所以
A fetch request does not return duplicate objects. It returns all objects that match the predicate. So
[NSPredicate predicateWithFormat: @"objectID = %@", oid]
[NSPredicate predicateWithFormat: @"(objectID = %@) OR (objectID = %@)", oid, oid]
[NSPredicate predicateWithFormat: @"objectID IN %@", @[oid, oid]]
返回具有给定对象ID(如果有)的所有一个对象.
return all one object with the given object ID (if there is one).
在您的情况下，您已经具有要显示的对象.因此，我建议将它们存储在数组中，并将其用作表视图数据源，而不要使用获取的结果控制器.
In your case, you have already the objects that you want to display. Therefore I would suggest to store them in an array and use that as table view data source, instead of using a fetched results controller.
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