how to prevent another where chain from quering db again - ruby-on-rails-4

Question

I have an object called work_shift_stops created with .where and after modified to limit start and end_time, with code below:
work_shift_stops = Stoppage20.where(["machine20_id = :machine20_id AND (end_time >= :start AND start <= :end_time)",
{ machine20_id: params[:shift_result][:machine20_id],
end_time: work_shift.end,
start: work_shift.first}])
work_shift_stops.each do |ws|
ws.start = work_shift.first if ws.start < work_shift.first
ws.end_time = work_shift.end if ws.end_time > work_shift.end
end
Further in my code I am using another where chain on the object:
load_time = open_time - work_shift_stops.where(stop20_id: Stop20.where(stop_group20_id: 3).pluck(:id)).sum(:duration)
This gives unpleasant result as rails is making another db query, which retrieves work_shift_stops with unmodified start and end_time limits:
SELECT SUM("stoppage20s"."duration") AS sum_id FROM "stoppage20s" WHERE (machine20_id = '1' AND (end_time >= '2013-03-02 05:00:00.000000' AND start <= '2013-03-02 17:00:00.000000')) AND "stoppage20s"."stop20_id" IN (12, 17)
Question is how to prevent rails from making another db query and use object with modified attributes?

Answer 1

What I actually did was dropping .sum(:duration) function and iterating on active record collection work_shift_stopswith modified start and end_time:
work_shift_stops.each do |wsd|
planned_stops_time += wsd.duration if Stop20.where(stop_group20_id: 3).pluck(:id).include? wsd.stop20_id
tech_stops_time += wsd.duration if Stop20.where(stop_group20_id: 1).pluck(:id).include? wsd.stop20_id
org_stops_time += wsd.duration if Stop20.where(stop_group20_id: 2).pluck(:id).include? wsd.stop20_id
end