核心数据:多对多关系的 NSPredicate.(“此处不允许使用多对多键") [英] Core Data: NSPredicate for many-to-many relationship. ("to-many key not allowed here")

问题描述

我有两个名为类别"和文章"的实体，它们具有多对多的关系.我想形成一个谓词，它搜索所有 category.name 等于某个值的文章.我有以下几点:

I have two entities named "Category" and "Article" which have a many to many relationship. I want to form a predicate which searches for all articles where category.name is equal to some value. I have the following:

 NSEntityDescription  *entityArticle   = [NSEntityDescription entityForName:@"Article" inManagedObjectContext:managedObjectContext]; 
 NSSortDescriptor  *sortDescriptor   = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];
 NSArray     *sortDescriptors  = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
 NSPredicate    *predicate    = [NSPredicate predicateWithFormat:@"categories.name == [cd] %@", category.name]; 

 [request setSortDescriptors:sortDescriptors];
 [request setEntity:entityArticle];
 [request setPredicate:predicate];

 NSMutableArray *results = [[managedObjectContext executeFetchRequest:request error:nil] mutableCopy];

 if ([results count] > 0)
  NSLog(@"Results found."); 
 else 
  NSLog(@"NO results found."); 

 [request release];
 [sortDescriptor release];
 [sortDescriptors release];

我收到的错误是 *** 由于未捕获的异常NSInvalidArgumentException"而终止应用程序，原因:此处不允许使用对多键"

是否有任何选项可以检索所需的数据?

Are there any options to retrieve the desired data?

推荐答案

您正在尝试将集合 (categories.name) 与标量值 (category.name).您需要使用集合比较器 (CONTAINS)，或使用谓词修饰符 (ANY/ALL/SOME> 等).

You're trying to compare a collection (categories.name) to a scalar value (category.name). You need to either use a collection comparator (CONTAINS), or use a predicate modifier (ANY/ALL/SOME, etc).

尝试使用:

[NSPredicate predicateWithFormat:@"ANY categories.name =[cd] %@", category.name];

或者:

[NSPredicate predicateWithFormat:@"categories.name CONTAINS[cd] %@", category.name];

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