Mongoid Hash field Issue

24 Apr 2012 - Bangalore, India

Mongodb is one of the most popular document-based databases due to it’s simplicity of use and speed.

I’ve been working with mongodb for more than a year now and it’s been quite awesome. No useless mysql queries, no joins, no “mysql-scalability” problems. It supports Hash and Array field types, has replication out-of-the-box, has a cool master-slave configuration which just works (unlike riak, which just seems confused).

Mongoid is my ODM choice as when I started out, Mongoid fully supported Rails 3 and used ActiveModel etc. whereas MongoMapper was still using the “validatable” gem, which meant they were inexplicably behind the curve.

To be honest, inspite of Mongoid being as brilliant as it is, it is still a work a progress. There are still issues, some of which are quite major ones.

Here’s one such issue I faced a while back.

Issue

Modifying a Hash that lives in enumerable data has a weird behaviour. Updating a Hash value does not seem to be possible although Mongoid does not throw out an error.

class SomeClass
  include Mongoid::Document
  field :test, :type => Hash
end

sc = SomeClass.create:)test => {:value => {}})

sc.test["value"]["new_hash"] = "new_value"

sc.save!

# Looks Fine
sc.test["value"]

# Reloading Object
sc = SomeClass.last
sc.test["value"]

# WTF! Where did my value go!
# => {}

Reason

When test is called from the sc object, Mongoid does a #dup and send you that copy. So, when you update the Hash, both these are updated and when Mongoid wants to save the Hash, it “thinks” that there is no change as both the copy and actual values are the same.

Workaround

Doing a deep copy will fix this issue. This will have a performance hit but if you want to update your Hash, you need to do this.

I found this reusable code in one if the Mongoid Issues:

In config/initializers/service.rb:

require 'service'
class Object
  include Service::DeepCopyable
end

In lib/service.rb:

module Service
  module DeepCopyable
    def grab_copy
      Marshal.load(Marshal.dump(self))
    end
  end
end

You can save your Hash now.

class Registrant
  include Mongoid::Document

  field :rgt_key, type: String
  field :nf_registrant, type: Hash

  before_save :check_keys

  def check_keys
    if self.rgt_key_changed?
      self.nf_registrant = self.nf_registrant.grab_copy
      self.nf_registrant["Registrant"]["reg_rgt_key"] = self.rgt_key
    end
  end
end