new column in ActiveRecord object, method_missing() called intermittently

rubyonrails-talk
1

Hi,

I'm using the geokit plugin to provide a "find-locations-on-radius" feature in my app. When a location is added to the database, the actual address is translated (geocoded) into a longitude & latitude. The longitude and the latitude are stored in the database. When a user performs a "find-locations-on-a-X-miles-radius" from a point of origin (an address), a distance is calculated from the given point of origin to all the stored locations. Geokit overrides the ActiveRecord find method to include a distance column:

[:all, {:select=>"*, SQRT(POW(XXX*(XXX- locations.lat),2)+\n POW(XXX*(XXX-locations.lng),2)) \n AS distance", :order=>"distance asc"}]

Then from any view you can access the distance on the objects retrieved by find:

Location.find(:all,:origin =>[@ip_location.lat, @ip_location.lng]).each { |location|   location.distance }

The problem that I have is that location.distance returns empty/nil once in a while. From what I understand, when location.distance is called, it invokes method_missing() in activerecord/lib/active_record/ attribute_methods.rb. That is because there is no distance column in the locations table, so it goes to the object's attributes to find the distance. Well, it just so happens that once in a while method_missing () is not invoked when accessing location.distance. I put tracing in method_missing() and when I get distance values method_missing was indeed called, and when I get blank/nil - method_missing was not called.

I thought it was because of caching, but I turned off caching in production, and I don't have any explicit caching logic in my views or in my controllers.

I asked this question on the geokit group but nobody answered it: http://groups.google.com/group/geokit/browse_thread/thread/ce364a46d802d399 . You can see all debugging and tracing here.

Why doesn't method_missing get invoked all the time when location.distance is called? What gets invoked then? Could this be some sort of caching bug?

Thanks, Jenna

2

Why doesn't method_missing get invoked all the time when location.distance is called? What gets invoked then? Could this be some sort of caching bug?

So the times that it returns nil, what does get called ? have you tried stepping through it with the debugger ?

Fred

3

The other issue I've seen with things like this was that some find options (:include, notably) will drop the :select clause, meaning that you won't get an attribute back for distance. Any chance that could be involved?

--Matt Jones

4

It returns nil when you use joins or includes on your finds. Try records.sort_by_distance_from origin. That will add the distance column back

5

I'll try that. It's just that it is hard to make it repeatable for debugging.

6

I did read that in the geokit README, but I have no include in my statement.

7

I'll try it. Thanks. Jenna

8

Hi Mike,

I tried your suggestion (putting sort_by_distance_from right after the find()) and it seems to work. I don't understand why, because I really don't have an :include in the find(). I'm just disappointed with myself because I thought I can debug any Rails problem, and this one showed me that I can't.

Thanks, J