Cocoa Core数据高效的方法来计算实体！ [英] Cocoa Core Data efficient way to count entities!

问题描述

我读了很多关于核心数据..但什么是一个有效的方式来计算一个实体类型（像SQL可以做SELECT count（1）...）。现在我只是解决这个任务与选择所有与NSFetchedResultsController和获取计数的NSArray！我相信这不是最好的方法...

谢谢

解决方案

我不知道是否使用NSFetchedResultsController是最有效的方式来实现你的目标（但它可能是）。获取实体实例计数的显式代码如下：

  //假设NSManagedObjectContext * moc 
 
 NSFetchRequest * request = [[NSFetchRequest alloc] init]; 
 [request setEntity：[NSEntityDescription entityForName：entityName inManagedObjectContext：moc]]; 
 
 [request setIncludesSubentities：NO]; //省略子实体。默认值为YES（即包含子实体）
 
 NSError * err; 
 NSUInteger count = [moc countForFetchRequest：request error：& err]; 
 if（count == NSNotFound）{
 //处理错误
} 
 
 [request release]; 
  

i read much about Core Data.. but what is an efficient way to make a count over an Entity-Type (like SQL can do with SELECT count(1) ...). Now i just solved this task with selecting all with NSFetchedResultsController and getting the count of the NSArray! I am sure this is not the best way...

thank you

解决方案

I don't know whether using NSFetchedResultsController is the most efficient way to accomplish your goal (but it may be). The explicit code to get the count of entity instances is below:

// assuming NSManagedObjectContext *moc

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:moc]];

[request setIncludesSubentities:NO]; //Omit subentities. Default is YES (i.e. include subentities)

NSError *err;
NSUInteger count = [moc countForFetchRequest:request error:&err];
if(count == NSNotFound) {
  //Handle error
}

[request release];

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