Efficient way to update multiple records with independent values? - sql

Question

I have the following (overly db expensive) method:
def reorder_area_routes_by_demographics!
self.area_routes.joins(:route).order(self.demo_criteria, :proximity_rank).readonly(false).each_with_index do |area_route, i|
area_route.update_attributes(match_rank: i)
end
end
But this results in an UPDATE query for each area_route. Is there a way to do this in one query?
--Edit--
Final solution, per coreyward suggestion:
def reorder_area_routes_by_demographics!
sorted_ids = area_routes.joins(:route).order(self.demo_criteria, :proximity_rank).pluck(:'area_routes.id')
AreaRoute.update_all [efficient_sort_sql(sorted_ids), *sorted_ids], {id: sorted_ids}
end
def efficient_sort_sql(sorted_ids, offset=0)
offset.upto(offset + sorted_ids.count - 1).inject('match_rank = CASE id ') do |sql, i|
sql << "WHEN ? THEN #{id} "
end << 'END'
end

Answer 1

I use the following to do a similar task: updating the sort positions of a bevy of records according to their order in params. You might need to refactor or incorporate this differently to accomodate the scopes you're applying, but I think this will send you in the right direction.
def efficient_sort_sql(sortable_ids, offset = 1)
offset.upto(offset + sortable_ids.count - 1).reduce('position = CASE id ') do |sql, i|
sql << "WHEN ? THEN #{i} "
end << 'END'
end
Model.update_all [efficient_sort_sql(sortable_ids, offset), *sortable_ids], { id: sortable_ids }
sortable_ids is an array of integers representing the ids of each object. The resulting SQL looks something like this:
UPDATE pancakes SET position = CASE id WHEN 5 THEN 1 WHEN 3 THEN 2 WHEN 4 THEN 3 WHEN 1 THEN 4 WHEN 2 THEN 5 WHERE id IN (5,3,4,1,2);
This is, ugliness aside, a pretty performant query and (at least in Postgresql) will either fully succeed or fully fail.