objective-c - NSFetchedResultsController 使用子属性排序

Question

我有一对多的父子实体书<->>借

书

• 姓名
• 借(关系)

借用

• 借用日期
• 注意
• 书(关系)

我想使用子实体的最大 NSDate 对 NSFetchedResultsController 中的 Book 实体进行排序，以显示最近借过的书。

我该怎么做？我尝试使用我自己的方法，例如使用 Book 上的类别

- (NSArray *)borrowSortedByborrowedDate
{
    NSSortDescriptor *sortByCreatedDate = [NSSortDescriptor sortDescriptorWithKey:@"borrowDate" ascending:NO];
    NSArray *sortedArray = [self.histories sortedArrayUsingDescriptors:[NSArray arrayWithObject:sortByCreatedDate]];
    return sortedArray;
}

- (NSDate *)recentBorrowDate
{
    Borrow *borrow = [[self borrowSortedByborrowedDate] objectAtIndex:0];
    return borrow.borrowDate;
}

并将其放入sortDescriptor

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"recentBorrowDate" ascending:YES];

但是没用。

这是我获取的请求(无效)

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

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Book" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

[fetchRequest setFetchBatchSize:20];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"recentBorrowDate" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];

我的预期结果是

• 书名1(借阅日期为今天)
• 书名2(昨天借的日期)
• Book name3 (borrowDate many days ago)

Answer 1

我有同样的问题，这是我目前实现所需输出的解决方法。 (也许这是唯一的方法，但我仍在寻找......)

当创建一个新的借阅时，只需同时更新 book.lastBorrowDate 属性。

// Create new Borrow and set Book
Borrow *newBorrow = (Borrow *)[NSEntityDescription insertNewObjectForEntityForName:@"Borrow" inManagedObjectContext:self.managedObjectContext];
newBorrow.book = theBook;
newBorrow.borrowDate = [NSDate date];
newBorrow.note = theNote;

// Now also update our book's lastBorrowDate tracking property
theBook.lastBorrowDate = [NSDate date];

// Save all our changes
NSError *error = nil;
if (![self.managedObjectContext save:&error]) {
    // Something went horribly wrong...
    NSLog(@"Unresolved error: %@, %@, %@", error, [error userInfo],[error localizedDescription]);
    abort(); // Fail
}

现在，稍后将它们拉出时很容易...

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

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Book" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

[fetchRequest setFetchBatchSize:20];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"lastBorrowDate" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];