Calculating columns with Core Data like SQL

Question

Hey,

I am sorting stores to get the closest stores. Furthermore I want to know how far away this shop is. The SQL query would look like this:

select *,

6371 * 2 * ATAN2(SQRT(POWER(SIN((RADIANS(?) - RADIANS(latitude))/2),2) + COS(RADIANS(latitude)) * COS(RADIANS(?)) * POWER(SIN((RADIANS(?) - RADIANS(longitude))/2),2)), SQRT(1-POWER(SIN((RADIANS(?) - RADIANS(latitude))/2),2) + COS(RADIANS(latitude)) * COS(RADIANS(?)) * POWER(SIN((RADIANS(?) - RADIANS(longitude))/2),2))) AS DISTANCE

FROM stores
ORDER BY DISTANCE

these question marks would be replaced by the longitude and latitude of the users position. (latitude, latitude, longitude, latitude, latitude, longitude)

How can I achieve this with iPhone SDK using Core Data?

Answer 1

From what I know of Core Data (at least the ORM) part of it, there is not a way to do aggregate functions or mathematical functions like what you're attempting. One has the ability to do predicates which equate to the WHERE clause, but not selection functions.

I believe that one could bypass Core Data and do that in sqlite.

Jack

Answer 2

CoreData itself can't really do this, because distanceFromCurrentLocation here would be a transient property (and you can't fetch on transient properties). But you can still do this in a natural way by fetching the objects into an NSMutableArray, and then sorting that array based on a distanceFromCurrentLocation transient property.

Depending on how often you do this and how often the user moves far enough to require recalculation, it could be more efficient to occasionally recalculate a -currentDistance persistent property, and then using simple predicates. This is a very efficient solution if you search much more often the user moves, because you can easily keep a single locationOfLastRecalculation persistent variable and only recalculate once when the user moves sufficiently. Since this would persist over restarts, the overall performance improvement over dynamic calculation could be substantial.