Average as a fetched property?

Question

I'm not sure if I've understood Fetched Properties correctly, so do tell me if I'm going the wrong way with this and should be doing something else instead.

I have a little demo app where I have books that are being reviewed. A Review entity has a score property with the Int16 from 1 to 5 and a timestamp, and the Book entity has zero-to-many reviews through the isReviewed property (bookInReview as its inverse property).

I want to make a fetched property on book that is an average review score from the past 30 days. Can I use fetched properties in my model in Xcode 4 to do this? If so, how? I can tried setting Destination to Review and predicate to @avg(score) to start with all the reviews but that doesn't seem right.

Cheers

Nik

Answer 1

Fetched Properties return an NSArray (actually an _NSFaultingArray, or something like that) of NSManagedObjects, so I don't believe it is possible to fetch aggregates like @sum, @average, or @count as fetched properties. However, that doesn't mean you cannot add them to a subclass of NSManagedObject or a category.