对核心数据属性/列执行基本计算

我使用的是基于核心数据的时间序列(实体/表),它由以下属性/列组成:

dateTime mc nd re

基于公式 result = (mc + nd) / re 我想计算每一行的结果,并在数组中返回结果(dateTime+value)。

我尝试了以下代码在数据库级别上执行计算,但在结果数组中确实得到了错误的结果(每行有多个dateTime值,计算结果错误)。然而 NSExpressionDescription NSLogging时返回正确的公式。

你知道我的代码可能出了什么问题吗?

NSString *divisorKeyPath = @"revenue"
NSExpression *marketCapExpression = [NSExpression expressionForKeyPath:@"mc"];
NSExpression *netDebtExpression   = [NSExpression expressionForKeyPath:@"nd"];
NSExpression *divisorExpression   = [NSExpression expressionForKeyPath:
    divisorKeyPath];
NSExpression *evExpression        = [NSExpression expressionForFunction:
    @"add:to:"  
    arguments:@[marketCapExpression,netDebtExpression]];
NSExpression *divisionExpression  = [NSExpression expressionForFunction:
    @"divide:by:"
    arguments:@[evExpression, divisorExpression]];
NSString *divisionExpressionName = @"division";
NSExpressionDescription *divisionDescription  = [[NSExpressionDescription alloc] init];
divisionDescription.name                      = divisionExpressionName;
divisionDescription.expression                = divisionExpression;
divisionDescription.expressionResultType      = NSDoubleAttributeType;

NSFetchRequest *divisionRequest = [NSFetchRequest fetchRequestWithEntityName:
   NSStringFromClass([SharePrice class])];
divisionRequest.propertiesToFetch = @[@"dateTime",divisionDescription];
divisionRequest.resultType = NSDictionaryResultType;
NSSortDescriptor *sortDescriptor = [NSSortDescriptor 
   sortDescriptorWithKey:@"dateTime" ascending:YES];
divisionRequest.sortDescriptors = @[sortDescriptor];

NSError *error;
NSArray *divisionResults = [self.managedDocument.managedObjectContext
   executeFetchRequest:divisionRequest error:&error];
NSLog(@"results: %@",divisionResults);