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

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 h开发者_如何转开发ave a tutorial or example of this?

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You would use the @sum collection operator thusly:

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