所有实例的核心数据总和属性

问题描述

使用核心数据，我遇到了一个问题。我有一个实体 Movement，属性为 amount。如何计算所有实例的所有数量之和？我想了解如何使用 NSExpressionDescription ，但是 NSSet 就足够了。

Using Core Data, I encountered a problem. I have an entity "Movement" with an attribute "amount". How do I make the sum of all the "amount" of all instances? I'd like to understand how to use NSExpressionDescription, but it's good enough NSSet.

推荐答案

具有managedObjectContext：

Having a managedObjectContext:

NSManagedObjectContext *managedObjectContext = ...

我们使用返回类型字典创建获取请求：

We create a fetch request with return type dictionary:

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([Movement class])];
fetchRequest.resultType = NSDictionaryResultType;

然后我们创建一个表达式描述来计算总和：

Then we create an expression description to calculate the sum:

NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
expressionDescription.name = @"sumOfAmounts";
expressionDescription.expression = [NSExpression expressionForKeyPath:@"@sum.amount"];
expressionDescription.expressionResultType = NSDecimalAttributeType;

设置请求属性以获取：

fetchRequest.propertiesToFetch = @[expressionDescription];

如果需要，我们还可以设置谓词。

We could also set a predicate if we want.

最后，我们执行请求，并得到一个包含一个带有一个键的字典的数组（@ sumOfAmounts），其值是一个带有数量总和的NSNumber。

And last we execute the request and get an array containing a dictionary with one key(@"sumOfAmounts") and its value is a NSNumber with the sum of amounts.

NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (result == nil)
{
    NSLog(@"Error: %@", error);
}
else
{
    NSNumber *sumOfAmounts = [[result objectAtIndex:0] objectForKey:@"sumOfAmounts"];
}

干杯

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