getting the sum of all instances of Entity B from a given instance of Entity A

问题

I have a to-many relationship between Entity A and Entity B (iOS Core Data app). I need to get the sum of the NSDecimalNumber attribute of all instances of Entity B from a given instance of Entity A. Does anyone have a tutorial or example of this?

回答1:

You would use the @sum collection operator thusly:

NSNumber *theSum=[ObjectA.objectBs valueForKey:@"@sum.numericalAttribute"];

来源：https://stackoverflow.com/questions/5251255/getting-the-sum-of-all-instances-of-entity-b-from-a-given-instance-of-entity-a