DeltaAttributes

This gem makes updating specified number fields by ActiveRecord in unusual way.

Instead of generating sql script to update value in usual way like this:

  UPDATE users
  SET money = 10
  WHERE id = 1;

It replaces it with

  UPDATE users
  SET money = money + delta
  WHERE id = 1;

where delta is difference between old value and new value of that field.

This solves problem with simultaneous updating of same field by different threads
without locking record (problem known as race condition http://en.wikipedia.org/wiki/Race_condition).

What it means for you is that you do NOT need to LOCK users table record when updating money field
from different threads if you "mark" money field as delta attribute. Sometimes this improves application
speed significantly as other threads don't wait till current thread finishes it's work.

Installation

Add this line to your application's Gemfile:

gem 'delta_attributes'

And then execute:

$ bundle

Or install it yourself as:

$ gem install delta_attributes

Usage

To mark numeric field to be updated by "field = field + delta" way you just need to add field name to delta_attributes like this:

class User < ActiveRecord::Base

  delta_attributes :money

end

Now you can:

u = User.first # money = 3, id = 1
u.money = 5
u.save

rails will generate

UPDATE users SET money = money + 2 where id = 1

and if

u = User.first # money = 5, id = 1
u.money = 1
u.money += 3
u.save

rails will generate

UPDATE users SET money = money - 1 where id = 1

You can use any operations with numeric field like:

u.money -= 10

u.money += 7

u.money = u.money * 2

etc.

It doesn't matter as delta is calculated as difference between original value received from database and new value before update.

Tested with rails 4.0, 3.2.9, 3.2.8, 3.2.0, 3.1.0

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request