问题
I have a Game model with a points attribute, and I want to compute the sum of the top 20 scores.
I can pluck the points and have ruby compute the sum, as in:
Game.order('points desc').limit(20).pluck(:points).sum
But I am curious as to whether there is a straightforward way to have AR produce an SQL aggregate calculation to accomplish the same. The following naive attempt does not work:
Game.sum(:points, order: 'points desc', limit: 20)
SELECT SUM(`games`.`points`) FROM `games`
Thanks
回答1:
Try this:
Game.order('points desc').limit(20).sum(:points)
回答2:
The limit() fails in the above answer
Game.order('point desc').limit(2).pluck(:point).inject(:+)
回答3:
I am going to answer my own question, but will gladly award a better solution.
The sum function really only takes one argument, and while AR 4.x used to simply ignore any extra args, the new AR 5.0.0 raises an error. The original attempt was indeed naive.
Following @pitabas' suggestion, I then tried:
Game.order('points desc').limit(20).sum(:points)
SELECT SUM("games"."points") FROM "games" LIMIT ? [["LIMIT", 20]]
As you can tell from the SQL constructed by AR, this does not work either, because the LIMIT clause acts on the result of the query, which is a single value. Furthermore, the ORDER clause is ignored altogether. The result of the above expression is the total sum of the points column, for all records.
An SQL query that actually works is something like:
SELECT SUM(points) FROM (SELECT points FROM games ORDER BY points DESC LIMIT 20)
The closest I got to that is the following:
sq = Game.order('points desc').limit(20)
Game.from(sq).sum('"points"')
which yields:
SELECT SUM("points") FROM (SELECT "games".* FROM "games" ORDER BY points desc LIMIT ?) subquery [["LIMIT", 20]]
Hack: note the double set of quotes around the points. That's how I managed to enforce a literal interpretation of the sum argument. Without them, the query raises a "no such column" error.
回答4:
Do note that there is no point in ordering the results if your only interested in computing the sum hence then answer below would suffice
Game.limit(10).sum(: points)
来源：https://stackoverflow.com/questions/37020161/activerecord-calculations-on-subsets-or-records