如何以编程方式创建NSFetchedPropertyDescription？

问题描述

我有一个预先存在的NSManagedObjectModel，我用XCode GUI创建。我想创建一个排序的抓取属性，XCode 3.2的GUI不支持。我在创建我的NSPersistentStoreCoordinator之前做所有这些，因为我知道你不能修改一个NSManagedObjectModel后，对象图管理器开始使用它。我创建了NSFetchedPropertyDescription这样：

I have a pre-existing NSManagedObjectModel that I created with the XCode GUI. I want to create a sorted fetched property, which XCode 3.2's GUI doesn't support. I do all of this before creating my NSPersistentStoreCoordinator because I know you can't modify a NSManagedObjectModel after an object graph manager has started using it. I created the NSFetchedPropertyDescription thusly:

NSManagedObjectModel *managedObjectModel = ... // fetch from my mainBundle

NSEntityDescription *fetchedPropertyEntityDescription = [entitiesByName objectForKey:@"MyEntity"];

NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:fetchedPropertyEntityDescription];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"myPredicateProperty == $FETCH_SOURCE"]];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"mySortProperty" ascending:YES]]];

NSFetchedPropertyDescription *fetchedPropertyDescription = [[[NSFetchedPropertyDescription alloc] init] autorelease];
[fetchedPropertyDescription setFetchRequest:fetchRequest];
[fetchedPropertyDescription setName:@"myFetchedProperty"];

NSEntityDescription *entityDescription = [entitiesByName objectForKey:@"MyFetchSourceEntity"];

[entityDescription setProperties:[[entityDescription properties] arrayByAddingObject:fetchedPropertyDescription]];

当我调用

[fetchedPropertyDescription setFetchRequest:fetchRequest];

我得到以下异常：

NSInvalidArgumentException: Can't use fetch request with fetched property description (entity model mismatch).

推荐答案

我需要在设置之前将NSFetchedPropertyDescription添加到NSEntityDescription NSFetchedPropertyDescription上的NSFetchRequest。

I needed to add the NSFetchedPropertyDescription to the NSEntityDescription before setting the NSFetchRequest on the NSFetchedPropertyDescription.

正确的步骤如下：

NSManagedObjectModel *managedObjectModel = ... // fetch from my mainBundle

NSEntityDescription *fetchedPropertyEntityDescription = [entitiesByName objectForKey:@"MyEntity"];

NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:fetchedPropertyEntityDescription];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"myPredicateProperty == $FETCH_SOURCE"]];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"mySortProperty" ascending:YES]]];

NSFetchedPropertyDescription *fetchedPropertyDescription = [[[NSFetchedPropertyDescription alloc] init] autorelease];
//DON'T DO THIS HERE, AN ERROR WILL OCCUR
//[fetchedPropertyDescription setFetchRequest:fetchRequest];
//
[fetchedPropertyDescription setName:@"myFetchedProperty"];

NSEntityDescription *entityDescription = [entitiesByName objectForKey:@"MyFetchSourceEntity"];

[entityDescription setProperties:[[entityDescription properties] arrayByAddingObject:fetchedPropertyDescription]];

//DO THIS HERE INSTEAD
[fetchedPropertyDescription setFetchRequest:fetchRequest];

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