Sometimes in your Ruby on Rails models, you want to override the attribute accessors to do additional processing on the values on their way in and out of the data storage layer. Custom serialization is a common use of this.

For example, when using ActiveRecord for your Rails models, you can provide custom attribute accessors, say to serialize a Hash to JSON, using the read_attribute and write_attribute methods like this:

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# Assume a text column named 'stuff'
class User < ActiveRecord::Base
  def stuff
    JSON.parse(read_attribute(:stuff))
  end

  def stuff=(new_val)
    write_attribute(:stuff, new_val.json)
  end
end

With this, you can assign a Hash to the stuff attribute of User, and when you access it via User#stuff, you get a Hash back. All the while, it’s read and written to and from the database as a JSON string.

Doing it with Ripple on top of Riak

Ripple is the Ruby modeling layer for the distributed NoSQL store, Riak. It tries very hard to provide a lot of the same interfaces as ActiveRecord. However, this is one of the areas it diverges: Ripple::Document objects do not support the read_attribute and write_attribute methods.

Instead, they implement the [] and []= methods. Translating the code above to work with Ripple is pretty easy:

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class User
  include Ripple::Document

  property :stuff, Hash

  def stuff
    JSON.parse(self[:stuff])
  end

  def stuff=(new_val)
    self[:stuff] = new_val.to_json
  end
end

Bonus Points

Using this tactic, you can easily add some memoization so that your getter doesn’t need to parse the JSON text on every access. To do this, we’ll use an instance variable as a cache that we’ll invalidate in the setter, like so:

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class User
  include Ripple::Document

  property :stuff, Hash

  def stuff
    @cached_stuff ||= JSON.parse(self[:stuff])
  end

  def stuff=(new_val)
    @cached_stuff = nil
    self[:stuff] = new_val.to_json
  end
end