删除核心数据中的对象 [英] Delete an object in core data

问题描述

我有一个实体在我的核心数据模型像这样：

  @interface选择：NSManagedObject 
 
 @property（nonatomic，retain）NSString * book_id; 
 @property（nonatomic，retain）NSString * contenu; 
 @property（nonatomic，retain）NSNumber * page_id; 
 @property（nonatomic，retain）NSNumber * nbrOfOccurences; 
 @property（nonatomic，retain）NSString * next; 
 @property（nonatomic，retain）NSString * previous; 
  

我创建了许多 Selection 他们在核心数据，现在我想删除一些选择与一些标准。例如，如果符合以下条件，我想删除 Selection 对象：

  content = test 
 page_id = 5 
 book_id = 1331313 
  

我可以做这个？

解决方案

Mike Weller 写的是正确的。

首先，您需要创建一个 NSFetchRequest ，如下所示：

  NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init]; 
 [fetchRequest setEntity：[NSEntityDescription entityForName：@SelectioninManagedObjectContext：context]]; 
  

然后您必须为该请求设置谓词，如下所示：

[fetchRequest setPredicate：[NSPredicate predicateWithFormat：@content ==％@ AND page_id ==％@ AND book_id ==％@，contentVal，pageVal， bookVal]];


其中

  NSString * contentVal = @test; 
 NSNumber * pageVal = [NSNumber numberWithInt：5]; 
 NSString * bookVal = @1331313; 
  

我使用％@

现在，您可以使用上一个请求在上下文中执行提取操作：

  NSError * error = nil; 
 NSArray * results = [context executeFetchRequest：fetchRequest error：& error]; 
  

results 包含所有匹配的托管对象

最后，您可以抓取对象并在其上调用删除。

  [context deleteObject：currentObj]; 
  

完成后，您需要按照文档保存上下文。

正如在保存上下文之前一个新对象不保存到商店中一样，在保存上下文之前不会从商店中删除已删除的对象。

因此

  NSError * error = nil; 
 [context save：& error]; 
  

请注意， save 方法返回一个bool值。因此，您可以使用类似以下的方法或向用户显示警报。来源 NSManagedObjectContext保存错误。

  NSError * error = nil; 
 if（[context save：& error] == NO）{
 NSAssert（NO，@Save should not fail \ n％@，[error localizedDescription]）; 
 abort（）; 
} 
  

I have an entity in my core data model like this:

@interface Selection : NSManagedObject

@property (nonatomic, retain) NSString * book_id;
@property (nonatomic, retain) NSString * contenu;
@property (nonatomic, retain) NSNumber * page_id;
@property (nonatomic, retain) NSNumber * nbrOfOccurences;
@property (nonatomic, retain) NSString * next;
@property (nonatomic, retain) NSString * previous;

I have created many Selections and saved them in Core Data and now I would like to delete some selections with some criteria. For example, I would like to delete a Selection object if matches the following:

content = test
page_id = 5
book_id = 1331313

How I can do this?

解决方案

What Mike Weller wrote is right. I'll expand the answer a little bit.

First you need to create a NSFetchRequest like the following:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];    
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Selection" inManagedObjectContext:context]];

Then you have to set the predicate for that request like the following:

[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"content == %@ AND page_id == %@ AND book_id == %@", contentVal, pageVal, bookVal]];

where

NSString* contentVal = @"test";
NSNumber* pageVal = [NSNumber numberWithInt:5];
NSString* bookVal = @"1331313";

I'm using %@ since I'm supposing you are using objects and not scalar values.

Now you perform a fetch in the context with the previous request:

NSError* error = nil;
NSArray* results = [context executeFetchRequest:fetchRequest error:&error];

results contains all the managed objects that match that predicate.

Finally you could grab the objects and call a deletion on them.

[context deleteObject:currentObj];

Once done you need to save the context as per the documentation.

Just as a new object is not saved to the store until the context is saved, a deleted object is not removed from the store until the context is saved.

Hence

NSError* error = nil;
[context save:&error];

Note that save method returns a bool value. So you can use an approach like the following or display an alert to the user. Source NSManagedObjectContext save error.

NSError *error = nil;
if ([context save:&error] == NO) {
    NSAssert(NO, @"Save should not fail\n%@", [error localizedDescription]);
    abort();
}

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