核心数据：删除实体的所有实例的最快方式 [英] Core Data: Quickest way to delete all instances of an entity

问题描述

我使用Core Data在本地保存来自Web服务调用的结果。 Web服务返回完整的对象模型，比方说Cars - 可能大约有2000个（而且我不能使Web服务返回小于1或所有的车辆。

I'm using Core Data to locally persist results from a Web Services call. The web service returns the full object model for, let's say, "Cars" - could be about 2000 of them (and I can't make the Web Service return anything less than 1 or ALL cars.

下一次我打开我的应用程序，我想刷新核心数据持久拷贝通过调用所有汽车的Web服务，但是为了防止重复，我需要先清除本地缓存中的所有数据。

The next time I open my application, I want to refresh the Core Data persisted copy by calling the Web Service for all Cars again, however to prevent duplicates I would need to purge all data in the local cache first.

有更快的方法来清除托管对象上下文中特定实体的所有实例（例如CAR类型的所有实体），或者我需要查询他们调用，然后迭代结果删除每个，然后保存？

Is there a quicker way to purge ALL instances of a specific entity in the managed object context (e.g. all entities of type "CAR"), or do I need to query them call, then iterate through the results to delete each, then save?

理想情况下，我可以说删除所有的实体是Blah。

Ideally I could just say delete all where entity is Blah.

推荐答案

iOS 9及更高版本：

iOS 9添加了一个新类 NSBatchDeleteRequest 允许你轻松地删除与谓词匹配的对象，而无需将它们全部加载到内存中。

iOS 9 and later:

iOS 9 added a new class called NSBatchDeleteRequest that allows you to easily delete objects matching a predicate without having to load them all in to memory. Here's how you'd use it:

let fetchRequest = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

do {
    try myPersistentStoreCoordinator.executeRequest(deleteRequest, withContext: myContext)
} catch let error as NSError {
    // TODO: handle the error
}

Objective-C

Objective-C

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];

NSError *deleteError = nil;
[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];

有关批量删除的详细信息，请参阅WWDC 2015的核心数据新功能（从〜14：10开始）。

More information about batch deletions can be found in the "What's New in Core Data" session from WWDC 2015 (starting at ~14:10).

全部提取并删除全部：

NSFetchRequest *allCars = [[NSFetchRequest alloc] init];
[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];
[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectID

NSError *error = nil;
NSArray *cars = [myContext executeFetchRequest:allCars error:&error];
[allCars release];
//error handling goes here
for (NSManagedObject *car in cars) {
  [myContext deleteObject:car];
}
NSError *saveError = nil;
[myContext save:&saveError];
//more error handling here

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