最有效的方法是获得平均值,同时过滤掉一些对象

Question

我试图获取子实体中属性的平均值,同时还尝试仅包含一组选择的记录.

我的Core Data模型中有两个实体:Invoice和InvoiceDetail.

Invoice:<br>
  invoiceNum - attribute<br>
  invoiceDate - attribute<br>
  invoiceDetails - one-to-many relationship to InvoiceDetail

InvoiceDetail:<br>
  itemAmount - attribute<br>
  itemType - attribute<br>
  invoice - one-to-one relationship to Invoice<br>

如果我想获得整个发票的itemAmount的平均值,我会使用以下(invoice是NSManagedObject):

float avgAmount = [[invoice valueForKeyPath:@"invoiceDetails.@avg.itemAmount"] floatValue];

但是,我只想获取itemType = 1的对象的平均值.我可以遍历invoiceDetail项目并手动执行此操作,但我知道这会导致性能问题.我不确定这样做的最佳方法是什么.

谢谢你的帮助.

Answer 1

您可以使用包含表达式的获取请求来执行此操作,如下所示:

- (NSDictionary *)myFetchResults
{
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    request.entity = [NSEntityDescription entityForName:@"InvoiceDetail" inManagedObjectContext:myContext];
    request.predicate = [NSPredicate predicateWithFormat:@"itemType = %@", [NSNumber numberWithInt:1]];

    request.resultType = NSDictionaryResultType;

    NSExpressionDescription *aveExDescr = [[NSExpressionDescription alloc] init];
    [aveExDescr setName:@"myAverage"];
    [aveExDescr setExpression:[NSExpression expressionForFunction:@"average:" 
                                                        arguments:[NSArray arrayWithObject:
                                                                   [NSExpression expressionForKeyPath:@"itemAmount"]]]];
    [aveExDescr setExpressionResultType:NSFloatAttributeType];

    request.propertiesToFetch = [NSArray arrayWithObject:aveExDescr];

    NSError *err = nil;
    NSArray *results = [self.moContext executeFetchRequest:request error:&err];
    [request release];
    [err release];

    return results;
}

fetch将返回一个字典,您可以按如下方式访问该字典:

NSArray *results = [self myFetchResults];
NSDictionary *resultsDictionary = [results lastObject];
NSNumber *average = [resultsDictionary objectForKey:@"myAverage"]; 

请注意,此代码尚未经过测试.NSDecimalAttributeType如果您正在使用NSDecimalNumbers,也可以使用而不是float类型.