Spira

It's time to breathe life into your linked data.

Synopsis

Spira is a framework for using the information in RDF.rb repositories as model objects. It gives you the ability to work in a resource-oriented way without losing access to statement-oriented nature of linked data, if you so choose. It can be used either to access existing RDF data in a resource-oriented way, or to create a new store of RDF data based on simple defaults.

An introductory blog post is at http://blog.datagraph.org/2010/05/spira

A changelog is available in the {file:CHANGES.md} file.

Example

class Person < Spira::Base

  configure :base_uri => "http://example.org/example/people"

  property :name, :predicate => FOAF.name, :type => String
  property :age,  :predicate => FOAF.age,  :type => Integer

end

bob = RDF::URI("http://example.org/people/bob").as(Person)
bob.age  = 15
bob.name = "Bob Smith"
bob.save!

bob.each_statement {|s| puts s}
#=> RDF::Statement:0x80abb80c(<http://example.org/example/people/bob> <http://xmlns.com/foaf/0.1/name> "Bob Smith" .)
#=> RDF::Statement:0x80abb8fc(<http://example.org/example/people/bob> <http://xmlns.com/foaf/0.1/age> "15"^^<http://www.w3.org/2001/XMLSchema#integer> .)

Features

• Extensible validations system
• Extensible types system
• Easy to use multiple data sources
• Easy to adapt models to existing data
• Open-world semantics
• Objects are still RDF.rb-compatible enumerable objects
• No need to put everything about an object into Spira
• Easy to use a resource as multiple models

ActiveModel integration

This is a version of Spira that makes use of ActiveModel. The goal of this version is to replace all the internals of Spira with ActiveModel hooks, and thus get rid of superfluous code and increase compatibility with Rails stack. I want it to be a drop-in replacement for ActiveRecord or any other mature ORM solution they use with Ruby on Rails.

Although I've been trying to make the impact of this transition to be as little as possible, there are a few changes that you should be aware of:

• Read the comments on "new_record?" and "reload" methods. They are key methods in understanding how Spira is working with the repository. Basically, a Spira record is new, if the repository has no statements with this record as subject. This means, that the repository is queried every time you invoke "new_record?". Also note that if Spira.repository is not set, your Spira resource will always be "new". Also note that instantiating a new Spira resource sends a query to the repository, if it is set, but should work just fine even if it's not (until you try to "save" it).
• Customary Rails' record manipulation methods are preferred now. This means, you should use more habitual "save", "destroy", "update_attributes", etc. instead of the "save!", "destroy!", "update", "update!" and others, as introduced by the original Spira gem.
• Callbacks are now handled by ActiveModel. Previous ways of defining them are no longer valid. This also introduces the "before_", "after_" and "around_" callbacks as well as their "_validation", "_save", "_update" and "_create" companions for you to enjoy.
• Validations are also handled by ActiveModel. With all the helper methods you have in ActiveRecord.
• A spira resource (class) must be defined by inheriting it from Spira::Base. Using "include Spira::Resource" is temporarily broken, but will be back at some point, with improvements and stuff.
• "after/before_create" callbacks are not called when only the properties of your Spira resource are getting persisted. That is, you may create a "type"-less Spira resource, assign properties to it, then #save it -- "_create" callbacks will not be triggered, because Spira cannot infer a resource definition ("resource - RDF.type - type") for such resource and will only persist its properties. Although this is how the original Spira behaves too, I thought I'd state it explicitly here before you start freaking out.
• Configuration options "base_uri", "default_vocabulary" and "repository_name" are now configured via "configure" method (see the examples below).
• A couple of (not so) subtle changes:
  1. Global caching is gone. This means that "artist.works.first.artist" (reverse lookup) does not return the original artist, but its copy retrieved from the database.

Getting Started

The easiest way to work with Spira is to install it via Rubygems:

$ sudo gem install spira

Downloads will be available on the github project page, as well as on Rubyforge.

Defining Model Classes

To use Spira, define model classes for your RDF data. Spira classes include RDF, and thus have access to all RDF::Vocabulary classes and RDF::URI without the RDF:: prefix. For example:

require 'spira'

class CD < Spira::Base
  configure :base_uri => 'http://example.org/cds'
  property :name,   :predicate => DC.title,   :type => XSD.string
  property :artist, :predicate => URI.new('http://example.org/vocab/artist'), :type => :artist
end

class Artist < Spira::Base
  configure :base_uri => 'http://example.org/artists'
  property :name, :predicate => DC.title, :type => XSD.string
  has_many :cds,  :predicate => URI.new('http://example.org/vocab/published_cd'), :type => XSD.string
end

Then use your model classes, in a way more or less similar to any number of ORMs:

cd = CD.for("queens-greatest-hits")
cd.name = "Queen's greatest hits"
artist = Artist.for("queen")
artist.name = "Queen"

cd.artist = artist
cd.save!
artist.cds = [cd]
artist.save!

queen = Artist.for('queen')
hits = CD.for 'queens-greatest-hits'
hits.artist == artist == queen

URIs and Blank Nodes

Spira instances have a subject, which is either a URI or a blank node.

A class with a base URI can instantiate with a string (or anything, via to_s), and it will have a URI representation:

Artist.for('queen')

However, a class is not required to have a base URI, and even if it does, it can always access classes with a full URI:

nk = Artist.for(RDF::URI.new('http://example.org/my-hidden-cds/new-kids'))

If you have a URI that you would like to look at as a Spira resource, you can instantiate it from the URI:

RDF::URI.new('http://example.org/my-hidden-cds/new-kids').as(Artist)
# => <Artist @subject=http://example.org/my-hidden-cds/new-kids>

Any call to 'for' with a valid identifier will always return an object with nil fields. It's a way of looking at a given resource, not a closed-world mapping to one.

You can also use blank nodes more or less as you would a URI:

remix_artist = Artist.for(RDF::Node.new)
# => <Artist @subject=#<RDF::Node:0xd1d314(_:g13751060)>>
RDF::Node.new.as(Artist)
# => <Artist @subject=#<RDF::Node:0xd1d314(_:g13751040)>>

Finally, you can create an instance of a Spira projection with #new, and you'll get an instance with a shiny new blank node subject:

formerly_known_as_prince = Artist.new
# => <Artist @subject=#<RDF::Node:0xd1d314(_:g13747140)>>

Class Options

A number of options are available for Spira classes.

base_uri

A class with a base_uri set (either an RDF::URI or a String) will use that URI as a base URI for non-absolute for calls.

Example CD.for 'queens-greatest-hits' # is the same as... CD.for RDF::URI.new('http://example.org/cds/queens-greatest-hits')

type

A class with a type set is assigned an RDF.type on creation and saving.

class Album < Spira::Base
  type URI.new('http://example.org/types/album')
  property :name,   :predicate => DC.title
end

rolling_stones = Album.for RDF::URI.new('http://example.org/cds/rolling-stones-hits')
# See RDF.rb at http://rdf.rubyforge.org/RDF/Enumerable.html for more information about #has_predicate?
rolling_stones.has_predicate?(RDF.type) #=> true
Album.type #=> RDF::URI('http://example.org/types/album')

In addition, one can count the members of a class with a type defined:

Album.count  #=> 1 

It is possible to assign multiple types to a Spira class:

class Man < Spira::Base
  type RDF::URI.new('http://example.org/people/father')
  type RDF::URI.new('http://example.org/people/cop')
end

All assigned types are accessible via "types":

Man.types #=> #<Set: {#<RDF::URI:0xd5ebc0(http://example.org/people/father)>, #<RDF::URI:0xd5e4b8(http://example.org/people/cop)>}>

Also note that "type" actually returns a first type from the list of types.

property

A class declares property members with the property function. See Property Options for more information.

has_many

A class declares list members with the has_many function. See Property Options for more information.

default_vocabulary

A class with a default_vocabulary set will transparently create predicates for defined properties:

class Song < Spira::Base
  configure :default_vocabulary => URI.new('http://example.org/vocab'),
            :base_uri => 'http://example.org/songs'
  property :title
  property :author, :type => :artist
end

dancing_queen = Song.for 'dancing-queen'
dancing_queen.title = "Dancing Queen"
dancing_queen.artist = abba
# See RDF::Enumerable for #has_predicate?
dancing_queen.has_predicate?(RDF::URI.new('http://example.org/vocab/title'))  #=> true
dancing_queen.has_predicate?(RDF::URI.new('http://example.org/vocab/artist')) #=> true

repository_name

Provides this class with a default repository to use instead of the :default repository if one is not set.

class Song < Spira::Base
  configure :repository_name => :songs
end

See 'Defining Repositories' for more information.

Property Options

Spira classes can have properties that are either singular or a list. For a list, define the property with has_many, for a property with a single item, use property. The semantics are otherwise the same. A has_many property will always return a list, including an empty list for no value. All options for property work for has_many.

property :artist, :type => :artist    #=> cd.artist returns a single value
has_many :cds,    :type => :cd        #=> artist.cds returns an array

Property always takes a symbol name as a name, and a variable list of options. The supported options are:

• :type: The type for this property. This can be a Ruby base class, an RDF::XSD entry, or another Spira model class, referenced as a symbol. See Types below. Default: Any
• :predicate: The predicate to use for this type. This can be any RDF URI. This option is required unless the default_vocabulary has been used.
• :localized: Indicates if the property is multilingual. See 'Localized Properties'

Localized Properties

A localized property allows to define a value per language. It only works with properties having a single item, ie defined with property.

class Article < Spira::Base
  property :label, :localized => true
end

# default locale :en
random_article = Article.for 'random-article'
random_article.label = "A label in english"
i18n.locale = :fr
random_article.label = "Un libellé en français"

random_article.label_native
# #=> [#<RDF::Literal:0xdb47c8("A label in english"@en)>, #<RDF::Literal:0xe5c3d8("Un libellé en français"@fr)>]

random_article.label_with_locales
# #=> {:en=>"A label in english", :fr=>"Un libellé en français"}

Types

A property's type can be either a class which includes Spira::Type or a reference to another Spira model class, given as a symbol.

Relations

If the :type of a spira class is the name of another Spira class as a symbol, such as :artist for Artist, Spira will attempt to load the referenced object when the appropriate property is accessed.

In the RDF store, this will be represented by the URI of the referenced object.

Type Classes

A type class includes Spira::Type, and can implement serialization and deserialization functions, and register aliases to themselves if their datatype is usually expressed as a URI. Here is the built-in Spira Integer class:

module Spira::Types
  class Integer

    include Spira::Type

    def self.unserialize(value)
      value.object
    end

    def self.serialize(value)
      RDF::Literal.new(value)
    end

    register_alias RDF::XSD.integer
  end
end

Classes can now use this particular type like so:

class Test < Spira::Base
  property :test1, :type => Integer
  property :test2, :type => RDF::XSD.integer
end

Spira classes include the Spira::Types namespace, where several default types are implemented:

• Integer
• Float
• Boolean
• String
• Any

The default type for a Spira property is Spira::Types::Any, which uses RDF::Literal's automatic boxing/unboxing of XSD types as best it can. See [RDF::Literal](http://rdf.rubyforge.org/RDF/Literal.html) for more information.

You can implement your own types as well. Your class' serialize method should turn an RDF::Value into a ruby object, and vice versa.

module MyModule
  class MyType
    include Spira::Type
    def self.serialize(value)
      ...
    end

    def self.unserialize(value)
      ...
    end
  end
end

class MyClass < Spira::Base
  property :property1, :type => MyModule::MyType
end

Defining Repositories

You can define multiple repositories with Spira, and use more than one at a time:

require 'rdf/ntriples'
require 'rdf/sesame'
Spira.add_repository! :cds,    RDF::Sesame::Repository.new 'some_server'
Spira.add_repository! :albums, RDF::Repository.load('some_file.nt')

class CD < Spira::Base
  configure :repository_name => :cds
end
class Album < Spira::Base
  configure :repository_name => :albums
end

Objects can reference each other cross-repository.

If no repository has been specified, the :default repository will be used.

repo = RDF::Repository.new
Spira.add_repository! :default, repo
Artist.repository == repo #=> true

Classes can specify a default repository to use other than :default with the repository_name function:

class Song < Spira::Base
  configure :repository_name => :songs
end

Song.repository #=> nil, won't use :default

Validations

[removed] See the description of ActiveModel::Validations.

Hooks

[removed] See the description of ActiveModel::Callbacks.

Using Model Objects as RDF.rb Objects

All model objects are fully-functional as RDF::Enumerable, RDF::Queryable, and RDF::Mutable. This lets you manipulate objects on the RDF statement level. You can also access attributes that are not defined as properties.

Support

There are a number of ways to ask for help. In declining order of preference:

• Fork the project and write a failing test, or a pending test for a feature request
• Ask on the public-rdf-ruby w3c mailing list
• You can post issues to the Github issue queue
• (there might one day be a google group or other such support channel, but not yet)

'License'

Spira is free and unemcumbered software released into the public domain. For more information, see the included UNLICENSE file.

Contributing

Fork it on Github and go. Please make sure you're kosher with the UNLICENSE file before contributing.