Cached attributes in ActiveRecord

This gem allows to save specific attributes in cache and save it to DB after limited time.

class Device < ActiveRecord::Base
  attr_cached :lat, :lon, by: :last_activity, expires_in: 5.minutes

  # Override setter (always call _cached method version!)
  def lat=(s)
    if s
      self.lat_cached = s
      super(s)
    end
  end
end

This means that parameters lat, lon, last_activity are persisted in Rails.cache. When the Device record is loaded from DB, those parameters are loaded from cache (if last_activity in DB is lower than last_activity in cache)... If you call save on user record and only lat, lon and last_activity parameters were changed, the record will be stored in DB only if last_activity stored in DB are older than 5 minutes!

Known issues with older ActiveRecords

There is a problem in dirty checking currently open in Rails! rails/rails#21442 Please see the dont reset dirty model after not save test to avoid current issue. We hope it will be fixed in new version of rails.

Cache control

If you want to store your model data in other specified cache, you can override attr_cached_key method in your model.

class Device
  attr_cached :lat, :lon

  def attr_cached_key
    "device_#{id}"
  end
end

Behaviour

2.1.5 :010 > d = Device.find 1
  Device Load (2.2ms)  SELECT  "devices".* FROM "devices"   ORDER BY "devices"."id" DESC LIMIT 1
 => #<Device id: 1, user_id: 548, lat: nil, lng: nil, last_activity: nil, created_at: "2015-09-25 14:24:45", updated_at: "2015-09-28 15:40:25">
2.1.5 :012 > d.lng = 10.302
 => 10.302
2.1.5 :013 > d.last_activity = Time.zone.now
 => Mon, 28 Sep 2015 21:01:06 CEST +02:00
2.1.5 :014 > d.save! # This will touch the DB!
   (0.3ms)  BEGIN
  SQL (3.4ms)  UPDATE "devices" SET "last_activity" = $1, "lng" = $2, "updated_at" = $4 WHERE "devices"."id" = 1  [["last_activity", "2015-09-28 19:01:06.955555"], ["lng", 10.203], ["updated_at", "2015-09-28 19:02:40.173626"]]
   (0.5ms)  COMMIT
 => true
2.1.5 :015 > d = Device.find 1
  Device Load (2.2ms)  SELECT  "devices".* FROM "devices"   ORDER BY "devices"."id" DESC LIMIT 1
 => #<Device id: 1, user_id: 548, lat: nil, lng: 10.2013, last_activity:  "2015-09-28 19:01:06.955555", created_at: "2015-09-25 14:24:45", updated_at: "2015-09-28 19:02:40.173626">
2.1.5 :016 > d.lat = 15.203
 => 15.203
2.1.5 :017 > d.last_activity = Time.zone.now.advance(seconds: 5)
 => Mon, 28 Sep 2015 21:04:42 CEST +02:00
2.1.5 :017 > d.save! # This will not!
   (0.3ms)  BEGIN
   (0.2ms)  COMMIT
 => true
2.1.5 :015 > d = Device.find 1
  Device Load (2.2ms)  SELECT  "devices".* FROM "devices"   ORDER BY "devices"."id" DESC LIMIT 1
 => #<Device id: 1, user_id: 548, lat: nil, lng: 10.2013, last_activity:  "2015-09-28 19:01:06.955555", created_at: "2015-09-25 14:24:45", updated_at: "2015-09-28 19:02:40.173626">
2.1.5 :016 > d.lat
=> 15.203
2.1.5 :017 > d.lat = 12.203
 => 12.203
2.1.5 :018 > d.last_activity = Time.zone.now.advance(minutes: 10)
 => Mon, 28 Sep 2015 21:14:42 CEST +02:00
2.1.5 :020 > d.save! # This will
   (0.3ms)  BEGIN
  SQL (3.4ms)  UPDATE "devices" SET "last_activity" = $1, "lng" = $2, "updated_at" = $4 WHERE "devices"."id" = 1  [["last_activity", "2015-09-28 19:14:42.952355"], ["lng", 12.203], ["updated_at", "2015-09-28 19:10:20.174329"]]
   (0.5ms)  COMMIT
 => true

Force save

If you really want from some reason write record to database and you've changed only cached values, set cache_force_save on object to true.

device = Device.first
device.lat = 5
device.cache_force_save = true
device.save! # This will update records in database (and each next save
on this object too!)

Instalation

gem 'attr_cached', github: 'netbrick/attr_cached'