使用CoreData执行乘法（聚合）：如何？ [英] Performing multiplication (aggregation) with CoreData: how to?

问题描述

Jeff Lamarche的精彩教程之后，我尝试汇总 NSManagedObject 的特定子类的数据。

这是场景。我创建了一个名为 Product 的类，它扩展了 NSManagedObject 类。 产品类有三个属性，如下所示：

  @property ，retain）NSString * name; 
 @property（nonatomic，retain）NSNumber * quantity; 
 @property（nonatomic，retain）NSNumber * price; 
  

我还创建了一个类别，名为 Product + Aggregate ，其中I执行和聚合。特别是，在Jeff教程之后，我管理了quantity属性的总和。

  +（NSNumber *）aggregateOperation：函数onAttribute：（NSString *）attributeName withPredicate：（NSPredicate *）predicate inManagedObjectContext：（NSManagedObjectContext *）context 
 {
 NSString * className = NSStringFromClass（[self class]）; 
 
 NSExpression * ex = [NSExpression expressionForFunction：function 
 arguments：[NSArray arrayWithObject：[NSExpression expressionForKeyPath：attributeName]]]; 
 
 NSExpressionDescription * ed = [[NSExpressionDescription alloc] init]; 
 [ed setName：@result]; 
 [ed setExpression：ex]; 
 [ed setExpressionResultType：NSInteger64AttributeType]; 
 
 NSArray * properties = [NSArray arrayWithObject：ed]; 
 [ed release]; 
 
 NSFetchRequest * request = [[NSFetchRequest alloc] init]; 
 [request setPropertiesToFetch：properties]; 
 [request setResultType：NSDictionaryResultType]; 
 
 if（predicate！= nil）
 [request setPredicate：predicate]; 
 
 NSEntityDescription * entity = [NSEntityDescription entityForName：className 
 inManagedObjectContext：context]; 
 [request setEntity：entity]; 
 
 NSArray * results = [context executeFetchRequest：request error：nil]; 
 NSDictionary * resultsDictionary = [results objectAtIndex：0]; 
 NSNumber * resultValue = [resultDictionary objectForKey：@result]; 
 
 return resultValue; 
} 
  

这个类方法从特定 UIViewController ：

  NSNumber * totalQuantity = [product aggregateOperation：@sum：onAttribute：@数量withPredicate：nil inManagedObjectContext：self.context]; 
  

代码运行良好。事实上，如果我说3个产品

  NAME QUANTITY PRICE 
产品1 2 23.00 
产品2 4 12.00 
 PRODUCT 3 1 2.00 
  

aggregateOperation 方法按预期返回7。

现在我再多一个步骤。修改该方法后，我需要返回产品订单的总成本。换句话说，我需要为每个产品计算QUANTITY * PRICE值，最后返回TOTAL。

你能建议我正确的方法吗？

这是Cyber​​fox建议之后使用的新代码，但很遗憾，它不起作用。

  NSString * className = NSStringFromClass（[self class]）; 
 
 NSArray * quantityPrice = [NSArray arrayWithObjects：[NSExpression expressionForKeyPath：@quantity]，[NSExpression expressionForKeyPath：@price]，nil]; 
 
 NSArray * multiplyExpression = [NSArray arrayWithObject：[NSExpression expressionForFunction：@multiply：by：arguments：quantityPrice]]; 
 
 NSExpression * ex = [NSExpression expressionForFunction：function arguments：multiplyExpression]; 
 
 NSExpressionDescription * ed = [[NSExpressionDescription alloc] init]; 
 [ed setName：@result]; 
 [ed setExpression：ex]; 
 [ed setExpressionResultType：NSInteger64AttributeType]; 
 
 //与之前相同
  

解决方案

以下是代码：

  static NSString * exprName1 = @val1; 
 static NSString * exprName2 = @val2; 
 
 NSString * className = NSStringFromClass（[self class]）; 
 
 NSExpression * quantityPathExpression = [NSExpression expressionForKeyPath：firstAttribute]; //例如。 quantity 
 NSExpression * unitaryPricePathExpression = [NSExpression expressionForKeyPath：secondAttribute]; //例如。价格
 
 NSExpressionDescription * quantityED = [[NSExpressionDescription alloc] init]; 
 [quantityED setName：exprName1]; 
 [quantityED setExpression：quantityPathExpression]; 
 [quantityED setExpressionResultType：NSDictionaryResultType]; 
 
 NSExpressionDescription * unitaryPriceED = [[NSExpressionDescription alloc] init]; 
 [unitaryPriceED setName：exprName2]; 
 [unitaryPriceED setExpression：unitaryPricePathExpression]; 
 [unitaryPriceED setExpressionResultType：NSDictionaryResultType]; 
 
 NSArray * properties = [NSArray arrayWithObjects：quantityED，unitaryPriceED，nil]; 
 [quantityED release]; 
 [unitaryPriceED release]; 
 
 NSFetchRequest * request = [[NSFetchRequest alloc] init]; 
 [request setPropertiesToFetch：properties]; 
 [request setResultType：NSDictionaryResultType]; 
 
 if（predicate！= nil）
 [request setPredicate：predicate]; 
 
 NSEntityDescription * entity = [NSEntityDescription entityForName：className inManagedObjectContext：context]; 
 [request setEntity：entity]; 
 
 NSError * error = nil 
 NSArray * results = [context executeFetchRequest：request error：& error]; 
 
 if（error！= nil）
 {
 NSLog（@发生错误：％@，[error localizedDescription]）; 
 abort（）; 
} 
 
 float total = 0; 
 for（NSDictionary * resultDict in results）
 {
 NSNumber * quantityNumber = [resultDict valueForKey：exprName1]; 
 NSNumber * unitaryPriceNumber = [resultDict valueForKey：exprName2]; 
 
 int moltVal = [quantityNumber intValue] * [unitaryPriceNumber intValue]; 
 
 total + = moltVal; 
} 
 
 return [NSNumber numberWithInt：total]; 
  

PS 使用它创建一个返回 NSNumber 并接受作为参数的托管上下文和要在其中执行数据检索的2个属性（ NSString ）。

希望它有帮助！

Following a fantastic tutorial by Jeff Lamarche, I'm trying to aggregate data for a specific subclass of NSManagedObject.

This is the scenario. I created a class named Product that extends NSManagedObject class. Product class has three properties like the following:

@property (nonatomic, retain) NSString* name;
@property (nonatomic, retain) NSNumber* quantity;
@property (nonatomic, retain) NSNumber* price;

I also created a category, called Product+Aggregate, where I perform a sum aggregation. In particular, following Jeff tutorial, I managed the sum for quantity attribute.

+(NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate inManagedObjectContext:(NSManagedObjectContext *)context
{
    NSString* className = NSStringFromClass([self class]);

    NSExpression *ex = [NSExpression expressionForFunction:function 
        arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:attributeName]]];

    NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
    [ed setName:@"result"];
    [ed setExpression:ex];
    [ed setExpressionResultType:NSInteger64AttributeType];

    NSArray *properties = [NSArray arrayWithObject:ed];
    [ed release];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setPropertiesToFetch:properties];
    [request setResultType:NSDictionaryResultType];

    if (predicate != nil)
        [request setPredicate:predicate];

    NSEntityDescription *entity = [NSEntityDescription entityForName:className
        inManagedObjectContext:context];
    [request setEntity:entity];

    NSArray *results = [context executeFetchRequest:request error:nil];
    NSDictionary *resultsDictionary = [results objectAtIndex:0];
    NSNumber *resultValue = [resultsDictionary objectForKey:@"result"];

    return resultValue;
}

This class method is called as follow from a specific UIViewController:

NSNumber *totalQuantity = [Product aggregateOperation:@"sum:" onAttribute:@"quantity" withPredicate:nil inManagedObjectContext:self.context];

The code works well. In fact, if I have say 3 product

NAME         QUANTITY      PRICE
PRODUCT 1    2             23.00 
PRODUCT 2    4             12.00
PRODUCT 3    1             2.00

The aggregateOperation method returns 7 as expected.

Now I would have one more step. Modifying that method, I need to return the total cost for product order. In other words, I need to calculate QUANTITY*PRICE value for each product and finally return the TOTAL.

Could you suggest me the right way? Thank you in advance.

EDIT This is the new code I use after Cyberfox suggestion but unfortunately it doesn't work.

NSString* className = NSStringFromClass([self class]);

NSArray *quantityPrice = [NSArray arrayWithObjects: [NSExpression expressionForKeyPath:@"quantity"], [NSExpression expressionForKeyPath:@"price"], nil];

NSArray *multiplyExpression = [NSArray arrayWithObject:[NSExpression expressionForFunction:@"multiply:by:" arguments:quantityPrice]];

NSExpression *ex = [NSExpression expressionForFunction:function arguments:multiplyExpression];

NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
[ed setName:@"result"];
[ed setExpression:ex];
[ed setExpressionResultType:NSInteger64AttributeType];

// same as before

解决方案

For those interested in, I found the solution for the problem above.

Here's the code:

static NSString* exprName1 = @"val1";
static NSString* exprName2 = @"val2";

NSString *className = NSStringFromClass([self class]); 

NSExpression *quantityPathExpression = [NSExpression expressionForKeyPath:firstAttribute]; //e.g. quantity    
NSExpression *unitaryPricePathExpression = [NSExpression expressionForKeyPath:secondAttribute]; //e.g. price

NSExpressionDescription *quantityED = [[NSExpressionDescription alloc] init];
[quantityED setName:exprName1];
[quantityED setExpression:quantityPathExpression];
[quantityED setExpressionResultType:NSDictionaryResultType];    

NSExpressionDescription *unitaryPriceED = [[NSExpressionDescription alloc] init];
[unitaryPriceED setName:exprName2];
[unitaryPriceED setExpression:unitaryPricePathExpression];
[unitaryPriceED setExpressionResultType:NSDictionaryResultType];

NSArray *properties = [NSArray arrayWithObjects:quantityED, unitaryPriceED, nil];
[quantityED release];
[unitaryPriceED release];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setPropertiesToFetch:properties];
[request setResultType:NSDictionaryResultType];

if (predicate != nil)
   [request setPredicate:predicate];

NSEntityDescription *entity = [NSEntityDescription entityForName:className inManagedObjectContext:context];
[request setEntity:entity];

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

if(error != nil)
{
   NSLog(@"An error occurred: %@", [error localizedDescription]);
   abort();
}

float total = 0;
for (NSDictionary *resultDict in results) 
{
   NSNumber* quantityNumber = [resultDict valueForKey:exprName1];
   NSNumber* unitaryPriceNumber = [resultDict valueForKey:exprName2];

   int moltVal = [quantityNumber intValue]*[unitaryPriceNumber intValue];

   total += moltVal;
}    

return [NSNumber numberWithInt:total];

P.S. to use it create a Class method that returns a NSNumber and accepts as parameters the managed context and 2 attributes (NSString) where you want to perform data retrieving.

Hope it helps!

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