GROUP BY与SUM（）使用Core Data [英] GROUP BY with SUM() using Core Data

问题描述

我遇到了类似的问题，但没有一个完整的例子说明如何完成。

我想翻译的SQL查询是： p>

  SELECT date，SUM（amount）FROM Table GROUP BY date; 
  

我需要帮助调试以下代码（目前fetchRequest返回nil）：

  entity = [NSEntityDescription entityForName：@TableinManagedObjectContext：self.managedObjectContext]; 
 [fetchRequest setEntity：entity]; 
 
 NSExpressionDescription * ex = [[NSExpressionDescription alloc] init]; 
 [ex setExpression：[NSExpression expressionWithFormat：@@ sum.amount]]; 
 [ex setExpressionResultType：NSDecimalAttributeType]; 
 
 [fetchRequest setPropertiesToFetch：[NSArray arrayWithObjects：@date，ex，nil]]; 
 [fetchRequest setPropertiesToGroupBy：[NSArray arrayWithObject：@date]]; 
 [fetchRequest setResultType：NSDictionaryResultType]; 
 
 [self.managedObjectContext executeFetchRequest：fetchRequest error：& error]; 
  

这是错误：

  2012-09-28 13：58：46.319 App [12205：c07] ***由于未捕获异常NSInvalidArgumentException终止应用程序，原因：'***  -  [__ NSArrayM insertObject：atIndex： ]：object can not be nil'
  

解决方案

  [ex setName：@somename]; 
 
 
 
 
在执行fetchRequest之前，文章：
 
 
 
  http：//iphonedevelopment.blogspot。 com / 2010/11 / nsexpression.html  
 
I came across similar questions but none of them had a complete example of how to accomplish it.

The SQL query I am trying to translate is this:
SELECT date, SUM(amount) FROM Table GROUP BY date;
I need help debugging the following piece code (currently the fetchRequest returns nil):
entity = [NSEntityDescription entityForName:@"Table" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

NSExpressionDescription* ex = [[NSExpressionDescription alloc] init];
[ex setExpression:[NSExpression expressionWithFormat:@"@sum.amount"]];
[ex setExpressionResultType:NSDecimalAttributeType];

[fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:@"date", ex, nil]];
[fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:@"date"]];
[fetchRequest setResultType:NSDictionaryResultType ];

[self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
This is the error:
2012-09-28 13:58:46.319 App[12205:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'

解决方案
The code above needs
[ex setName:@"somename"];
before executing the fetchRequest.

Useful article here:

http://iphonedevelopment.blogspot.com/2010/11/nsexpression.html

                        
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