getting the sum of all instances of Entity B from

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"];