Hungry Machine

Problem

With high volume Rails applications, entities with unique constraints are expensive and error prone to create/update. ActsAsInsertOrUpdate helps solve that problem (if you’re using MySQL), by leveraging the “INSERT … ON DUPLICATE KEY UPDATE” functionality.

Scenario

Lets say you have a Person, and Entity, and a Rating. Each user can rate each entity only once, and if they re-rate the entity, it should update the value.

class Entity < ActiveRecord::Base
  has_many :ratings
end

class Person < ActiveRecord::Base
 has_many :ratings
end

class Rating < ActiveRecord::Base
 belongs_to   erson
 belongs_to :Entity
end  

Here is the table that back’s Rating. Notice the Unique Key constraint on (entity_id, person_id).

CREATE TABLE `ratings` (
  `id` int(11) NOT NULL auto_increment,
  `rating` tinyint(4) default '0',
  `person_id` int(11) default NULL,
  `entity_id` int(11) default NULL,
  `created_at` datetime default NULL,
  `updated_at` datetime default NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `index_ratings_on_entity_id_and_person_id` (`entity_id`,`person_id`),
)

Previously, the logic would be something like:

• 1) Check if a rating exists for the User + Entity
• 2) If so, update
• 3) If not, insert
• 4) rescue the insert in case there is a unqiue constraint error
• 5) retrieve the record (and/or update with the new rating)

If the table is MyISAM, Steps 1-5 aren’t transactionally safe. If you’re using InnoDB, and experience heavy volumes of traffic, you’re prone to Deadlock’s. This is even more of a concern is the unique entity is shared across multiple users, as seen with a recent client of ours.

Solution:

class Rating < ActiveRecord::Base
 belongs_to   erson
 belongs_to :Entity
 acts_as_insert_or_update :field_to_update => "rating"
end  

Now Steps 1-5 above become, just one. Rating.create(..)

In the background, ActsAsInsertOrUpdate overwrites the implementation of ActionRecord:Base#create, to leverage an often unsed feature of MySQL called INSERT … ON DUPLICATE KEY UPDATE. As configured above, if a duplicate record is found for the unique constraint, the rating field will be updated with the new value.

Caution

This is a brute force hack on ActiveRecord::Base#create. Use at your own risk.

Code

Waiting for a rubyforge account. Will post more info soon.