如何获取核心数据中的一对多关系？ [英] How to fetch one to many relationship in core data?

问题描述

让我们说数据实体是： Bookshop< --->>书

Let say the data entities are: Bookshop <--->> Books

如何获取属于某个书店的所有书，例如名称中包含cloud？以下方法感觉笨重。

How do I fetch all books belonging to a specific book shop that contain "cloud" in the name for example? The following method feel clunky.

Bookshop *bookshop = (Bookshop *) nsManagedObjectFromOwner;
NSString *searchTerm = @"cloud";

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Books" 
                          inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat:
              @"ANY bookshop.Name == '%@' && Name contain[cd] '%@'", 
              bookshop.Name, searchTerm];
[fetchRequest setPredicate: predicate];

如果有两个非常相似的书店，我该如何改进这个代码？也可能性能改进的提取作为UITableView应该更新为用户类型的书名。

How could I improve this code to handle if there are 2 book shops with very similar names? And maybe also performance improvement for fetching as the UITableView should update as the user type in book name.

推荐答案

这可能是个人喜好的问题，但我喜欢定义和使用获取请求模板。然后你可以这样做：

This might be a matter of personal taste, but I like to define and use fetch request templates. Then you can do something like this:

Bookshop *bookshop = (Bookshop *) nsManagedObjectFromOwner;
NSString *searchTerm = @"cloud";

NSDictionary *substitutionDictionary = [NSDictionary dictionaryWithObjectsAndKeys:bookshop.Name, @"BOOKSHOP_NAME", searchTerm, @"BOOK_NAME", nil];
// BOOKSHOP_NAME and BOOK_NAME are the name of the variables in your fetch request template

NSFetchRequest *fetchRequest = [model fetchRequestFromTemplateWithName:@"YourFetchRequestTemplateName" substitutionVariables:substitutionDictionary];

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

这不是少的代码，但是不太笨重，因为所有的获取请求一个地方。或者我误解了你的问题？

It isn't less code, but it is less clunky because all of your fetch requests are defined in one place. Or maybe I'm misunderstanding your question?

至于你的问题的第二部分，我没有实际使用这个自己，但NSFetchedResultsControllerDelegate提供了方法，您需要自动保持结果表更新。尝试 -controller：didChangeObject：atIndexPath：forChangeType：newIndexPath：。显然它意味着使用NSFetchedResultsController。

As far as the second part of your question is concerned, I haven't actually used this myself, but the NSFetchedResultsControllerDelegate provides the methods you need to automatically keep your table of results up-to-date. Try -controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:. Obviously it implies using an NSFetchedResultsController.

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