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Note: I am no longer actively maintaining this module, and I do not know whether it is compatible with newer versions of mongoid. If you would like to take ownership of this repository, please let me know.

Helper module for denormalizing association attributes in Mongoid models. Why denormalize? Read A Note on Denormalization.

Extracted from coookin', where it is used in production.


Add the gem to your Bundler Gemfile:

gem 'mongoid_denormalize'

Or install with RubyGems:

$ gem install mongoid_denormalize


In your model:

# Include the helper method
include Mongoid::Denormalize

# Define your denormalized fields
denormalize :name, :email, :from => :user


class User
  include Mongoid::Document
  include Mongoid::Denormalize

  has_many :comments
  has_many :moderated_comments, :class_name => "Comment", :inverse_of => :moderator

  field :name
  field :email
  denormalize :name, :email, :to => :comments

class Comment
  include Mongoid::Document
  include Mongoid::Denormalize

  belongs_to :user
  belongs_to :moderator, :class_name => "User", :inverse_of => :moderated_comments

  field :body
  denormalize :name, :email, :from => :user

>> user = User.create(:name => "John Doe", :email => "")
>> comment = Comment.create(:body => "Lorem ipsum...", :user => user)
>> user.comments << comment
>> comment.user_name
"John Doe"
>> comment.user_email
>> user.update_attributes(:name => "Bill")
>> comment.user_name


Denormalization can happen in either or both directions. When using the :from option, the associated objects will fetch the values from the parent. When using the :to option, the parent will push the values to its children.

# Basic denormalization. Will set the user_name attribute of "self.comments" to the value of "".
denormalize :name, :to => :comments

# Multiple children. Will set the user_name attribute of "self.posts" and "self.comments" with "".
denormalize :name, :to => [:posts, :comments]

# Basic denormalization, obeying :inverse_of. Will set the moderator_name attribute of "self.moderated_comments"
denormalize :name, :email, :to => :moderated_comments

# With custom field name prefix. Will set the commenter_name attribute of "self.comments" (takes precedence over
# inverse_of).
denormalize :name, :to => :comments, :as => :commenter
denormalize :name, :from => :comments, :as => :commenter

# Basic denormalization. Will set the user_name attribute with the associated user's name.
denormalize :name, :from => :user

# Multiple fields. Will set the user_name and user_email attributes with the associated user's name and email.
denormalize :name, :email, :from => :user

# Basic denormalization. Will set the moderator_name attribute with the associated author user's name.
denormalize :name, :from => :moderator

When using :from, if the type of the denormalized field is anything but String (the default), you must specify the type with the :type option.

# in User
field :location, :type => Array
denormalize :location, :to => :posts

# in Post
denormalize :location, :type => Array, :from => :user

A few notes on behavior:

:from denormalizations are processed as before_save callbacks.

:to denormalizations are processed as after_save callbacks.

With :to, validations are not run on the object(s) containing the denormalized field(s) when they are saved.

Rake tasks

A rake task is included for verifying and repairing inconsistent denormalizations. This might be useful your records may be modified without Mongoid, or if you are using relational associations in combination with embedded records (see Known issues).

rake db:denormalize

This task will only update records that have outdated denormalized fields.

Known issues

Relational associations in combination with embedded records

It is not recommended nor supported to use mongoid_denormalize to perform denormalization to embedded records through a relational association because MongoDB/Mongoid do not support direct access to embedded fields via a relational association.

So, if User has_many :posts and User has_many :comments, but Comments are embedded_in :post, a user can't directly access a comment.


Clone the repository and run Bundler with bundle install. Use rake -T to view available rake tasks.

You can also use bundle exec guard for test driven development, if desired.

A MongoDB server is required to run the specs. Docker is an easy option:

docker run --name mongo3 -d -p 27017:27017 mongo



Copyright (c) 2010 Logan Raarup, released under the MIT license.