Rendering scopes with `includes` is slow for no reason

[giovannibonetti]

We have in our codebase something like this:

class AppointmentsController < BaseController
  # GET /api/v2/appointments
  def index
    appointments = Appointment.all.includes(:professional, :service)
    jsonapi_render json: appointments
  end
end

We use includes to eager load associations and avoid the N+1 query problem.
However, the record_count query becomes very slow since there is now a LEFT OUTER JOIN for each eager loded association:

SELECT DISTINCT COUNT(DISTINCT appointments.id) FROM "appointments" LEFT OUTER JOIN "professionals" ON "professionals"."id" = "appointments"."employment_id" LEFT OUTER JOIN "services" ON "services"."id" = "appointments"."service_id"

I suppose a simple fix could be done by changing this line from this:

def count_records(records, options)
  ...
  records.except(:group, :order).count("DISTINCT #{records.table.name}.id")
end

to this:

def count_records(records, options)
  ...
  records.except(:group, :includes, :order).count("DISTINCT #{records.table.name}.id")
end

By the way, can you please backport this patch to the 0.4 series as well?

[tiagopog]

Sure, I will work on a patch version bringing this improvement for both series 0.5 and 0.4. Thanks for reporting 👍

[tiagopog]

The update that fixes this issue was first packed in v0.7.1 but I'm still going to release it for v0.4.x.