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Embedding Mongoid documents and data migrations

When first starting out with mongodb, it’s easy to make the wrong decision on whether to embed a document or not. Even if you made the correct decision at that moment, changing requirements may force you into a migration. So how do you migrate existing data when transitioning from a standalone document to an embedded document? This is what I came up with.

Initial Data Structure

class User
  include Mongoid::Document
  field :name
  references_many :sales
end

class Sale
  include Mongoid::Document
  field :price, :type => Integer
  referenced_in :user
end

Now with Sale embedded in User

class User
  include Mongoid::Document
  field :name
  embeds_many :sales
end

class Sale
  include Mongoid::Document
  field :price, :type => Integer
  embedded_in :user, :inverse_of => :sales
end

Migrating Sales Data

class EmbedSalesInUsers < Mongoid::Migration
  def self.up

    # pull your existing data into memory
    # consider batching for large data sets
    # Note that you must call query methods on the object you are migrating
    # for this method to work (i.e. you can not pull via User#sales)

    sales_attributes = while_stand_alone_doc(Sale) do
      Sale.all.map(&:attributes)
    end

    # now when you save your data, your fields will be embedded

    sales_attributes.each do |attributes|
      user = User.find(attributes[:user_id])
      user.sales << Sale.new(:price => attributes[:price])
    end

    # remove all the documents from the original collection

    while_stand_alone_doc(Sale) do
      Sale.destroy_all
    end
  end

  def self.while_stand_alone_doc(klass)
    # by changing the Mongoid::Document.embedded you can temporarily
    # modify which collection Mongoid looks to for your model's data store

    begin
      klass.embedded = false

      yield
    ensure
      klass.embedded = true
    end
  end

end

There are a couple things to note here.

  • The embedded flag in Mongoid::Document is not documented so it could easily change. This was working as of 2.0.0.beta.20
  • When you create the new embedded document, make sure you pass only the attributes you care about. Passing all attributes will add things that you no longer need like user_id in this case. (For clarity, attributes you assign will be persisted, though you will only have setters and getters for the fields you explicitly define in your document.
  • I am using mongoid_rails_migrations in this example
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