Using a Ruby Library to Store Objects

Problem

You want to use an open source library to persist Ruby objects.

Solution

Explore your options and pick one! There exist several libraries to store and persist objects in Ruby. In the following recipe, we'll use Ohm and remodel as examples, but the reader should note that there are several good libraries that can be used for this purpose. [Please add more library examples with other libraries.]

A general abstraction strategy is to map objects without complicated schema definitions, and instead use a very simple domain-specific language to achieve similar results.

Consider the following example in Ohm. Here, we'll model three arbitrary objects (an Event, a Venue, and a Person), and provide validations for the Event.

require 'ohm'

class Event < Ohm::Model
  attribute :name
  reference :venue, Venue
  set :participants, Person
  counter :votes

  index :name

  def validate
    assert_present :name
  end
end

class Venue < Ohm::Model
  attribute :name
  collection :events, Event
end

class Person < Ohm::Model
  attribute :name
end

The Ruby libraries also employ a simple style to model associations. Here is an example of how to create a familiar 'has_many' relationship, using remodel.

require 'remodel'

class Cookbook < Remodel::Entity
  has_many :recipes, :class => 'Recipe', :reverse => :book
  property :title, :class => String
  property :author, :class => String
end

class Recipe < Remodel::Entity
  has_one :book, :class => Cookbook, :reverse => :recipes
  property :name, :class => String
end

If you store the above as cookbook.rb and have Redis running locally, then you can open a Ruby shell and do:

>> require 'cookbook.rb'
=> true
>> book = Cookbook.create :title => 'Python Cookbook', :author => 'Alex Martelli'
=> #<Cookbook(c:1) title: "Python Cookbook", author: "Alex Martelli">
>> recipe = book.recipes.create :name => 'Sorting a Dictionary'
=> #<Recipe(r:4) name: "Sorting a Dictionary">
>> recipe.book
=> #<Cookbook(c:1) title: "Python Cookbook", author: "Alex Martelli">

Discussion

An important reason to use an existing library to persist objects is the power of built-in assertions, or validations. Ohm, for example, includes assertions that ensure a field matches a certain format, or that the the field is numeric. Regular expressions can be used for the format validation:

assert_format :username, /^\w+$/

There are various strategies for storing object properties. For example, in remodel, all properties of an object are serialized into a JSON hash, which is stored under a single key. Associations are handled differently, though — both ends of the association are stored under separate keys. has_many uses a Redis list to store the keys of associated objects, has_one uses a Redis string to store the single associated key.

See Also