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So I have two objects, Invoice and InvoiceLineItem. InvoiceLineItem has a property called cost and it is dynamically created based on other properties. To help with the KVO/bindings I use: + (NSSet *)keyPathsForValuesAffectingCost { return [NSSet setWithObjects:@"lineItemType", @"serviceCost", @"hourlyRate", @"timeInSeconds", @"productCost", @"quantityOfProduct", @"mileageCost", @"milesTraveled", nil]; } This works great. When I edit a property like serivceCost the main cost in the Table View updates fine. In the Invoice object I have an NSMutableArray of InvoiceLineItems. Invoice has a similar property called totalCost . It is calculated by iterating over the line items and as long as the line item isn't marked as deleted(which I do for syncing reasons) it adds up the costs and creates the totalCost. Now my question/issue. How do I set up Invoice's totalCost so that it works with KVO/bindings when one of the line item's costs has changed? I tried setting up: + (NSSet *)keyPathsForValuesAffectingTotalCost { return [NSSet setWithObjects:@"lineItems.cost", nil]; } but it doesn't work. I end up with an error in the console: [<NSCFArray 0x1499ff40> addObserver:forKeyPath:options:context:] is not supported. Key path: cost

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