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 CHANGES file.

Example

class Person

  include Spira::Resource

  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

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
  include Spira::Resource
  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
  include Spira::Resource
  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
  include Spira::Resource
  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 

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
  include Spira::Resource
  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

default_source

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

class Song
  default_source :songs
end

See 'Defining Repositories' for more information.

validate

Provides the name of a function which does some sort of validation. See 'Validations' 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.

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 XSD.integer
  end
end

Classes can now use this particular type like so:

class Test
  include Spira::Resource
  property :test1, :type => Integer
  property :test2, :type => 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
  include Spira::Resource
  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')

CD.repository = :cds
Album.repository = :albums

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 default_source function:

class Song
  default_source :songs
end

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

Validations

You may declare any number of validation functions with the validate function. Before saving, each referenced validation will be run, and the instance's Spira::Errors object will be populated with any errors. You can use the built in assert and assert helpers such as assert_set and asssert_numeric.

class CD
  validate :is_real_music
  def is_real_music
    assert(artist.name != "Nickelback", :artist, "cannot be Nickelback")
  end

  validate :track_count_numeric
  def track_count_numeric
    assert_numeric(track_count)
  end
end

dancing_queen.artist = nickelback
dancing_queen.save!  #=> ValidationError
dancing_queen.errors.each #=> ["artist cannot be Nickelback"]

dancing_queen.artist = abba
dancing_queen.save!  #=> true

Hooks

Spira supports before_create, after_create, after_update, before_save, after_save, and before_destroy hooks:

class CD
  def before_save
    self.publisher = 'No publisher set' if self.publisher.nil?
  end
end

The after_update hook only fires on the update method, not simple property accessors (to allow you to easily set properties in these without going into a recursive loop):

class CD
  def after_update
    self.artist = 'Queen' # every artist should be Queen!
  end
end

# ...snip ...
dancing_queen.artist
#=> "ABBA"
dancing_queen.name = "Dancing Queen"
dancing_queen.artist
#=> "ABBA"
dancing_queen.update(:name => "Dancing Queen")
dancing_queen.artist
#=> "Queen"

Inheritance

You can extend Spira resources without a problem:

class BoxedSet < CD
  include Spira::Resource
  property cd_count, :predicate => CD.count, :type => Integer
end

You can also make Spira modules and include them into other classes:

module Media
  include Spira::Resource
  property :format, :predicate => Media.format
end

class CD
  include Spira::Resource
  include Media
end

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)

Authors, Development, and License

Authors

'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.