MongoSequence provides light-weight, robust sequences in MongoDB. Such sequences are useful for auto-incrementing or counting. Compatible with any Mongo ODM.

MongoSequence creates named sequences in a "sequences" collection in your database and atomically increments and returns the counter on them using Mongo's findAndModify command. You won't have collisions—two processes trying to increment at the same time will always get different numbers.

Install with Bundler:

gem "mongo_sequence"

Install without Bundler:

gem install mongo_sequence --no-ri --no-rdoc

If you're not using MongoMapper or Mongoid, you'll have to tell MongoSequence what database to use:

MongoSequence.database = Mongo::Connection.new.db('my_app_development')

Now, increment some sequences:

MongoSequence[:global].next    # => 1
MongoSequence[:global].next    # => 2

# get the current value...
# no guarantees of course on how long it's valid
# usually you use the return value of #next
MongoSequence[:global].current # => 2

# can also reset sequences if you need to
MongoSequence[:global] = 100
MongoSequence[:global].next    # => 101

# sequences with different names are independent
MongoSequence[:bluejay].next   # => 1
MongoSequence[:bluejay].next   # => 2

Here's how the sequences look in Mongo:

MongoSequence.collection.find_one(:_id => 'bluejay')
# =>
# {
#   "_id"     => "bluejay",
#   "current" => 2
# }

Why would anyone need atomically incrementing sequences with unique return values? Well, the most common case is for auto-incrementing id's in Mongo. Here's a MongoMapper example:

class Peregrine
  include MongoMapper::Document
  key :_id, Integer, :default => lambda { nil }

  before_create do
    self.id ||= MongoSequence[:peregrine_id].next # for id's unique among Peregrines
    # or
    self.id ||= MongoSequence[:mongo_id].next     # for id's unique across the database
  end
end

Need something added? Please open an issue! Or, even better, code it yourself and send a pull request:

# fork it on github, then clone:
git clone [email protected]:your_username/mongo_sequence.git
bundle install
rspec
# hack away
git push
# then make a pull request

All I've done here is merge the mongo2_fix branch into master so I can use it in my own projects.

A first implimentation was written over a year ago based on Chris Shiflett's Auto Increment with MongoDB blog post.

Authored by Brian Hempel. Public domain, no restrictions.