如何计数在coredata（聚合）？

本文介绍了如何计数在coredata（聚合）？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我正在学习核心数据，特别是在汇总工作。

当前我要做什么：计算表中的记录数在某些标准上具有反比关系的多对多关系。

目前我正在这样做：

NSExpression * ex = NSExpression expressionForFunction：@count： arguments：[NSArray arrayWithObject：[NSExpression expressionForKeyPath：@ddname]]]; NSPredicate * pred = [NSPredicate predicateWithFormat：@ddtype =='Home']; NSExpressionDescription * ed = [[NSExpressionDescription alloc] init]; [ed setName：@countDDEvents]; [ed setExpression：ex]; [ed setExpressionResultType：NSInteger16AttributeType]; NSArray * properties = [NSArray arrayWithObject：ed]; NSFetchRequest * request = [[NSFetchRequest alloc] init]; [request setPredicate：pred]; [request setPropertiesToFetch：properties]; [request setResultType：NSDictionaryResultType]; NSEntityDescription * entity = [NSEntityDescription entityForName：@DDEventinManagedObjectContext：[self.currentAccount managedObjectContext]]; [request setEntity：entity]; NSArray * results = [[self.currentAccount managedObjectContext] executeFetchRequest：request error：nil]; NSDictionary * dict = [results objectAtIndex：0]; NSLog（@女性英雄的平均出生日期：％@，[dict objectForKey：@countDDEvents]）;

来自 jeff lemarche。

EDIT ：我发现我的解决方案为

NSFetchRequest * request = [[NSFetchRequest alloc] init]; NSPredicate * pred = [NSPredicate predicateWithFormat：@ddtype =='Home']; [request setPredicate：pred]; NSEntityDescription * entity = [NSEntityDescription entityForName：@DDEventinManagedObjectContext：[self.currentAccount managedObjectContext]]; [request setEntity：entity]; NSError * error = nil; NSUInteger count = [[self.currentAccount managedObjectContext] countForFetchRequest：request error：& error];

它工作正常。但我想一次做更多的类型的请求。所以我认为这不是一个首选的计数方法。

EDIT ：

所以我认为这种方法是合适的一种????

所以任何人都可以告诉我更有效率的做这个的首选方法。 p>

感谢。

解决方案

Jeff LaMarche只是使用这个例子。在实践中，这种需求非常普遍，键值编码具有内置宏来处理它和其他常见的收集操作。

请参阅：键值编程指南：集合和数组运算符

在这种情况下，您将在您的谓词中使用 @count 运算符。

当然，手动调整自己的表达式可以精确控制谓词，但是操作符处理这种任务的80％。

I am learning core data and particularly working on aggregation.

Current what I want to do : count the number of records from the table which is in to-many relationship with inverse relationship on some criteria.

Currently I am doing this :

NSExpression *ex = [NSExpression expressionForFunction:@"count:" arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:@"ddname"]]]; NSPredicate *pred = [NSPredicate predicateWithFormat:@"ddtype == 'Home'"]; NSExpressionDescription *ed = [[NSExpressionDescription alloc] init]; [ed setName:@"countDDEvents"]; [ed setExpression:ex]; [ed setExpressionResultType:NSInteger16AttributeType]; NSArray *properties = [NSArray arrayWithObject:ed]; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setPredicate:pred]; [request setPropertiesToFetch:properties]; [request setResultType:NSDictionaryResultType]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"DDEvent" inManagedObjectContext:[self.currentAccount managedObjectContext]]; [request setEntity:entity]; NSArray *results = [[self.currentAccount managedObjectContext] executeFetchRequest:request error:nil]; NSDictionary *dict = [results objectAtIndex:0]; NSLog(@"Average birthdate for female heroes: %@", [dict objectForKey:@"countDDEvents"]);

Its from jeff lemarche.

EDIT : and I have found my solution as

NSFetchRequest *request = [[NSFetchRequest alloc] init]; NSPredicate *pred = [NSPredicate predicateWithFormat:@"ddtype == 'Home'"]; [request setPredicate:pred]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"DDEvent" inManagedObjectContext:[self.currentAccount managedObjectContext]]; [request setEntity:entity]; NSError *error = nil; NSUInteger count = [[self.currentAccount managedObjectContext] countForFetchRequest:request error:&error];

It is working nicely .But I want to do more request of such type at a time . So i think this can't be a preferred way of getting the count .

EDIT :

So I think the approach would be the appropriate one ????

So can anyone tell me more efficient an preferred way of doing this .

Thanks .

解决方案

Jeff LaMarche is just using this as a simple example. In practice, this need is so common that Key-Value Coding has a built in macro to handle it and other common collection operations.

See: The Key-Value Programming Guide: Set and Array Operators

In this case you would use the @count operator in your predicate.

Of course, hand tuning your own expression gives you fine control over your predicates but the operators handle 80% of such task.