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Addon for Mongoid to allow optimistic locking (CAS)

branch: master
README.md

mongoid_optimistic_locking

This gem helps to abstract the "Update if Current" method which may be used as a replacement for transactions in Mongo.

The gem is an addon over Mongoid ODM and is based on ActiveRecord's Optimistic Locking.

Compatibility

Works with Mongoid 3.

For Mongoid 2 use version 0.0.2

Rails 3 Installation

Add the gem to your Gemfile:

gem 'mongoid_optimistic_locking'

Usage

To use it, all you have to do is add include Mongoid::OptimisticLocking:

class Post
  include Mongoid::Document
  include Mongoid::OptimisticLocking
  field :text
end

This will add a _lock_version field in the document which will be incremented every time a save is called. Be sure to rescue Mongoid::Errors::StaleDocument to handle applicative logic in case the object was changed.

For example:

class PostController < ApplicationController
  ## Adds an "UPDATE: ...some text..." to an existing document
  def add_update
    begin
      post = Post.find(params[:id])
      post.text += "---UPDATE---  " + params[:more_text]
      post.save
    rescue Mongoid::Errors::StaleDocument
      retry
    end
  end
end

That's it!

Embedded Document Caveats

While Mongoid::OptimisticLocking can be used to some degree within embedded documents, there are certain limitations due to Mongoid's document embedding callback structure. Consider the following example:

class Post
  include Mongoid::Document
  field :text
  embeds_many :comments
end

class Comment
  include Mongoid::Document
  include Mongoid::OptimisticLocking
  embedded_in :post
  field :text
end

post = Post.new
comment = post.comments.build(:text => 'hello')
comment.save # will use optimistic locking checks
post.save # will not use optimistic locking checks

Open sourced by

Boxee

References

Mongo Developer FAQ - How do I do transactions/locking?

Mongo Atomic Operations - "Update if Current"

Presentation from "Startup Day" on Mongoid Optimistic Locking

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