:select and :include in ActiveRecord queries

Along with Bill Eisenhauer, I’ve been digging into what it would take to fix the problem I found with GeoKit, that it wouldn’t support :include queries properly. The explanation has seen us going deep into the internals of ActiveRecord to discover, as others have before, that the :select parameter (which GeoKit uses to build its distance column in the query) and the :include parameter don’t play nicely together.

The reason for that is that Rails employs special methods to build the select parameters when there’s a :include in the query (ie. when you’re joining with other tables and eager-loading them) and currently ignores :select along the way. GeoKit would ordinarily produce SQL along the lines of:

SELECT *,
	SQRT(
		POW(69.1*(42.962927-latitude), 2) +
		POW(42.0590342306803*(-85.637179-longitude), 2)
	) AS distance
FROM bus_stops
ORDER BY distance ASC LIMIT 1

but with a :include on a has_many relationship, we don’t get that distance column and so most of the queries will fail.

There are a couple of patches on the rails trac that seek to clean that up, but so far neither has been adopted. I’m sure the GeoKit developers would appreciate any input on workarounds.