ruby on rails - How to get recent n items average by ActiveRecord -

there company has many dailydatum. dailydatum has date , volume columns.

class company < activerecord::base   has_many :daily_data end 

i want average of recent 10 volumes. code got average value.(i implemented array#mean method)

company = company.first company.daily_data.order(date: :desc).limit(10).pluck(:volume).mean 

and thought it's more efficiant if use postgresql avg function. somehow order ignored this.

company.daily_data.order(date: :desc).limit(10).average(:volume)    (15.7ms)  select  avg("daily_data"."volume") "daily_data" "daily_data"."company_id" = $1 limit 10  [["company_id", 1123]] 

can use limit, order, average @ same time?

you need use sub-query queries that

select avg(volume)   (select volume daily_data company_id = 1 limit 10) temp 

doing in rails include find_by_sql method. don't think notice performance benefit this, since you're calculating 10 records.