How to: ORM/ODM Extensions
* Auditing and Versioning
  * [active_snapshot](https://github.com/westonganger/active_snapshot) - Simplified snapshots and restoration for ActiveRecord models and associations with a transparent white-box implementation
  * [acts_as_archival](https://github.com/expectedbehavior/acts_as_archival) - ActiveRecord plugin for atomic object tree archiving.
  * [ActsAsParanoid](https://github.com/ActsAsParanoid/acts_as_paranoid) - ActiveRecord plugin allowing you to hide and restore records without actually deleting them.
  * [Audited](https://github.com/collectiveidea/audited) - Audited is an ORM extension for ActiveRecord & MongoMapper that logs all changes to your models.
  * [Destroyed At](https://github.com/dockyard/ruby-destroyed_at) - Allows you to "destroy" an object without deleting the record or associated records.
  * [Discard](https://github.com/jhawthorn/discard) - A simple ActiveRecord mixin to add conventions for flagging records as discarded.
  * [Logidze](https://github.com/palkan/logidze) - Database changes log for Rails.
  * [marginalia](https://github.com/basecamp/marginalia) - Attach comments to your ActiveRecord queries. By default, it adds the application, controller, and action names as a comment at the end of each query.
  * [mongoid-history](https://github.com/aq1018/mongoid-history) - Multi-user non-linear history tracking, auditing, undo, redo for mongoid.
  * [PaperTrail](https://github.com/airblade/paper_trail) - Track changes to your ActiveRecord models' data for auditing or versioning.
  * [Paranoia](https://github.com/radar/paranoia) - A re-implementation of acts_as_paranoid for Rails 3 and 4, using much, much, much less code.
  * [PermenantRecords](https://github.com/JackDanger/permanent_records) - Soft-delete your ActiveRecord records, like an explicit version of ActsAsParanoid.
* Bit array
  * [ActiveFlag](https://github.com/kenn/active_flag) - Store up to 64 multiple flags in a single integer column with ActiveRecord.
  * [Bitfields](https://github.com/grosser/bitfields) - Save migrations and columns by storing multiple booleans in a single integer.
* Import
  * [ActiveRecord Import](https://github.com/zdennis/activerecord-import) - a library for bulk inserting data using ActiveRecord.
  * [bulk_insert](https://github.com/jamis/bulk_insert) - A little ActiveRecord extension for helping to insert lots of rows in a single insert statement.
  * [data_miner](https://github.com/seamusabshere/data_miner) - Download, pull out of a ZIP/TAR/GZ/BZ2 archive, parse, correct, and import XLS, ODS, XML, CSV, HTML, etc. into your ActiveRecord models.
  * [ferry](https://github.com/cmu-is-projects/ferry) - A ruby gem for easy data transfer.
* Misc
  * [arel_extensions](https://github.com/faveod/arel-extensions) - Extending Arel: more "rubyish" syntax, functions for strings, dates, math... and add native extensions for some DBs.
  * [ActiveRecord::Turntable](https://github.com/drecom/activerecord-turntable) - A database sharding extension for ActiveRecord.
  * [ActiveValidators](https://github.com/franckverrot/activevalidators) - An exhaustive collection of off-the-shelf and tested ActiveModel/ActiveRecord validations.
  * [DeepPluck](https://github.com/khiav223577/deep_pluck) - Allow you to pluck attributes from nested associations without loading a bunch of records.
  * [Enumerize](https://github.com/brainspec/enumerize) - Enumerated attributes with I18n and ActiveRecord/Mongoid/MongoMapper support.
  * [Goldiloader](https://github.com/salsify/goldiloader) - Automatic ActiveRecord eager loading.
  * [Rating](https://github.com/wbotelhos/rating) - A true Bayesian rating system with scope and cache enabled.
* Multi-tenancy
  * [Acts As Tennant](https://github.com/ErwinM/acts_as_tenant) - Add multi-tenancy to a Rails app through a shared database strategy.
  * [Apartment](https://github.com/influitive/apartment) - Multi-tenancy for Rails and ActiveRecord.
  * [Milia](https://github.com/jekuno/milia) - Non-invasive multi-tenancy for Rails which supports Devise authentication out of the box.
* Social
  * [Merit](https://github.com/merit-gem/merit) - Adds reputation behavior to Rails apps in the form of Badges, Points, and Rankings for ActiveRecord or Mongoid.
  * [PublicActivity](https://github.com/chaps-io/public_activity) - Provides easy activity tracking for your ActiveRecord, Mongoid 3 and MongoMapper models in Rails 3 and 4. Similar to Github's Public Activity.
  * [Simple Feed](https://github.com/kigster/simple-feed) - Fast and highly scalable read-optimized social activity feed library in pure Ruby, backed by Redis.
  * [Unread](https://github.com/ledermann/unread) - Manage read/unread status of ActiveRecord objects - and it's fast.
* Sorting
  * [ActsAsList](https://github.com/swanandp/acts_as_list) - Provides the capabilities for sorting and reordering a number of objects in a list.
  * [positioning](https://github.com/brendon/positioning) - Simple positioning for Active Record models. Su