Rails 4：使用.includes（：association）时修改热切负载查询

Question

I have two models:

class User < ActiveRecord::Base
  has_many :purchases

  # Perform joins and attach some calculations to the User object
  scope :add_stats, -> { group("users.id").joins(:purchases).select("users.*, SUM(purchases.price) AS total_purchases") }
end

class Purchase < ActiveRecord::Base
  belongs_to :user
end

The add_stats scope represents heavy calculations attached to the User objects. So if I want to get all User objects with stats, I just write User.all.add_stats .

So far so good. Now I want to fetch some Purchase objects and eager load the User s with stats as well. I've tried this:

belongs_to :user, -> { add_stats }

But then when Rails eager load the users, it seems to remove .group("user.id").joins(:purchases) and complain on purchases.price - "purchases table unknown". So the .select() is the only thing preserved from the scope.

How do I apply a scope (with working .group().joins() ) to the eager load query of all included belongs_to :user objects?

Answer 1

i just tried it in my rails4 app and it works

class User < ActiveRecord::Base
  scope :add_stats, -> { group("users.id").joins(:events).select("users.*, SUM(events.id) AS total_events") }

class Event
  belongs_to :user, -> { add_stats }

in rails console

Event.includes(:users).first.user.total_events
Reloading...
  Event Load (0.1ms)  SELECT "events".* FROM "events" WHERE "events"."label" = 'hamburg' ORDER BY "events"."id" ASC LIMIT 1
  Participant Load (0.3ms)  SELECT "participants".* FROM "participants" WHERE "participants"."event_id" IN (2)
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2, 3, 4, 8, 10, 11, 12, 14, 16, 17, 18, 19, 20)
  User Load (0.3ms)  SELECT users.*, SUM(events.id) AS total_events FROM "users" INNER JOIN "events" ON "events"."user_id" = "users"."id" WHERE "users"."id" = ? GROUP BY users.id ORDER BY "users"."id" ASC LIMIT 1  [["id", 2]]
=> 68

Event.first.user.total_events
Reloading...
  Event Load (0.2ms)  SELECT "events".* FROM "events" WHERE "events"."label" = 'hamburg' ORDER BY "events"."id" ASC LIMIT 1
  User Load (0.2ms)  SELECT users.*, SUM(events.id) AS total_events FROM "users" INNER JOIN "events" ON "events"."user_id" = "users"."id" WHERE "users"."id" = ? GROUP BY users.id ORDER BY "users"."id" ASC LIMIT 1  [["id", 2]]
=> 68

i guess that this is not what you want, as this does not use the scope for the include.