ActiveRecord query too difficult for my tiny brain - ruby-on-rails-3

Question

I have an activerecord model Event that I want to order by a rank. The rank will be a sum of weighted properties. For instance I might want to rank an event with some logic like:
LOG(number of followers) + (7 - number of days from now)
The following works, but is not satisfactory because it returns a result set instead of a relation object. Therefore I won't be able to treat it as a scope. (FYI I am using Postgres with PostGIS extension)
x = Event.find_by_sql("
SELECT
(
CASE WHEN COUNT(follows.*) = 0
THEN 0
ELSE LOG(COUNT(follows.*)) END
+
SIGN(7 - (EXTRACT(EPOCH FROM start) - EXTRACT(EPOCH FROM NOW())) / 86400) * LOG(ABS(7 - (EXTRACT(EPOCH FROM start) - EXTRACT(EPOCH FROM NOW())) / 86400))
) as score,
events.*
FROM events
LEFT OUTER JOIN follows
ON events.id = follows.followable_id AND follows.followable_type = 'Event' AND follows.blocked = 'f'
WHERE (events.start > NOW()) AND (ST_DWithin(st_setsrid(st_point(#{#location[:lng]}, #{#location[:lat]}), 4326), st_transform(loc, 4326), 48280.2, true))
GROUP BY events.id
ORDER BY 1 DESC
")
I understand that I could add a counter cache to the Events table and avoid the join, but in the future I will want to compute the rank through some other association so it would be very helpful to know how.
Thanks

Answer 1

This is actually pretty easy to split up into ActiveRecord::Relation queries.
x = Event.select("(
CASE WHEN COUNT(follows.*) = 0
THEN 0
ELSE LOG(COUNT(follows.*)) END
+
SIGN(7 - (EXTRACT(EPOCH FROM start) - EXTRACT(EPOCH FROM NOW())) / 86400) * LOG(ABS(7 - (EXTRACT(EPOCH FROM start) - EXTRACT(EPOCH FROM NOW())) / 86400))
) as score,
events.*")
x = x.joins("LEFT OUTER JOIN follows
ON events.id = follows.followable_id AND follows.followable_type = 'Event' AND follows.blocked = 'f'")
x = x.where("(events.start > NOW()) AND (ST_DWithin(st_setsrid(st_point(#{#location[:lng]}, #{#location[:lat]}), 4326), st_transform(loc, 4326), 48280.2, true))")
x = x.group('events.id')
x = x.order('1 desc')
Of course, I would recommend splitting these up in various scopes, but this should at least get you in the right direction.

Answer 2

I don't think this answer your question, but I want to use formatting.
I would make something easier, as I am more comfortable with ruby,I would create a new methods in your Event class:
def rank
# you rank computation according model columns and associations
Math.log(followers_count) + (7 - number_of_days_from_now)
end
def self.by_rank(order = 'desc')
all.sort{ |event, event1| order == 'desc' ? event1 <=> event : event <=> event1 }
end
Then you can scale caching your computation, creating a rank column in events table, so you can then do something like this:
Event.order('rank desc')
Event.order('rank asc')