如何创建基于属性和关系过滤Core Data对象的NSFetchRequest？ [英] How to create an NSFetchRequest which filters Core Data objects based on attributes AND relationships?

问题描述

我有一个Core Data模型设置如下：

I have a Core Data model setup like so:

• Blockbuster 实体
  
  
  
  
  • 与 DVD 实体有许多关系。
  
  
  • Blockbuster Entity
    • To-Many relationship to DVD entities.
    
    
    • 标题属性（字符串）
    
    
    • 与父级 Blockbuster 实体的一对一关系
    
    
    • title attribute (string)
    • To-One relationship to a parent Blockbuster entity

    一个大片可以有多个DVD里面（To-Many）。每个DVD只能是一个Blockbuster（To-One）的一部分。

    A single Blockbuster can have multiple DVD's inside of it (To-Many). Each DVD can only be part of a single Blockbuster (To-One).

    有多个Blockbuster的，有些可能包含与DVD实体具有相同标题的DVD实体从其他Blockbuster的。例如， 可能有一个变形金刚2的副本， 和 让我们假设没有Blockbuster有同一个标题电影的多个副本。

    There are multiple Blockbuster's, and some may contain DVD entities which share the same title as DVD entities from other Blockbuster's. For example, Blockbuster A might have a copy of "Transformers 2", and so does Blockbuster C and Blockbuster G. Let's pretend no Blockbuster has more than one copy of the same titled movie.

    我有兴趣从变形金刚2 > Blockbuster C 。我不想从 Blockbuster A 或 G 复制，因为我的老板经常在那里，我只能在燃烧这块垃圾时离开。

    I'm interested in grabbing the Transformers 2 DVD from Blockbuster C. I don't want the copies from Blockbuster A or G, because my boss frequents there, and I can only get away with burning this piece of garbage in Blockbuster C.

    我的问题是，如何形成NSFetchRequest， / strong>与标题Transformers 2（ Blockbuster C父实体的子级）之间的关系？

    My question is, how do I form an NSFetchRequest which grabs a DVD with the title "Transformers 2", which is a child of the Blockbuster "C" parent entity?

    此类型的请求将抓取所有Transformer 2DVD，但不是特定于Blockbuster C实体的请求：

    This type of request will grab all the "Transformer 2" DVD's, but not the one specific to the Blockbuster C entity:

    NSManagedObjectContext *moc = [self managedObjectContext];
    
    NSString *aTitle = @"Transformers 2";
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"title == %@", aTitle];
    [request setEntity:[NSEntityDescription entityForName:@"DVD" inManagedObjectContext:moc]];
    [request setPredicate:predicate];
    
    NSError *error = nil;
    NSArray *results = [moc executeFetchRequest:request error:&error];
    
    // error handling code
    [request release];
    

    有一种方法可以传递，比如说，NSPredicate中相关对象的Object ID，基于<？p>
    

    Is there a way to pass, say, the Object ID of a related object inside the NSPredicate, filtering based on that?

    推荐答案

    可以遍历 NSPredicate 中的关系。例如，您可以编写

    You can traverse relationships in an NSPredicate. For example, you could write something like

    [NSPredicate predicateWithFormat:@"title == %@ AND blockbuster.name LIKE \"Blockbuster C\"", @"Transformers 2"]
    

    有一个属性可以比较，你需要检查实际对象，然后你可以使用像

    Now, if you don't have a property to compare against and you need to check actual objects, then you could use something like

    [NSPredicate predicateWithFormat:@"title == %@ AND blockbuster IN %@", @"Transformers 2", setOfBlockbusters]
    

    完整语法记录在这里。但 setOfBlockbusters 可以是一个集合，一个数组或一个字典（如果它是一个字典，值，而不是键）。

    The full syntax is documented here. But setOfBlockbusters could be a set, an array, or a dictionary (if it's a dictionary, the values, not the keys, are used).

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