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RDF Serializers

About

RDF Serializers enables serialization to RDF formats. It uses fast-jsonapi serializers, with a few modifications. The serialization itself is done by the rdf gem.

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Installation

Add this line to your application's Gemfile:

gem 'rdf-serializers'

And then execute:

$ bundle

Getting started

First, register the formats you wish to serialize to. For example, add the following to config/initializers/rdf_serializers.rb:

require 'rdf/serializers/renderers'

RDF::Serializers::Renderers.register(:ntriples)

This automatically registers the MIME type.

In your controllers, add:

respond_to do |format|
  format.nt { render nt: model }
end

Configuration

You can configure the gem using RDF::Serializers.configure.

RDF::Serializers.configure do |config|
  config.always_include_named_graphs = false # true by default. Whether to include named graphs when the serialization format does not support quads.
  config.default_graph = RDF::URI('https://example.com/graph') # nil by default.
end

Formats

You can register multiple formats, if you add the correct gems. For example, add rdf-turtle to your gemfile and put this in the initializer:

require 'rdf/serializers/renderers'

opts = {
  prefixes: {
    ns:   'http://rdf.freebase.com/ns/',
    key:  'http://rdf.freebase.com/key/',
    owl:  'http://www.w3.org/2002/07/owl#',
    rdfs: 'http://www.w3.org/2000/01/rdf-schema#',
    rdf:  'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
    xsd:  'http://www.w3.org/2001/XMLSchema#'
  }
}

RDF::Serializers::Renderers.register(%i[ntriples turtle], opts)

The RDF gem has a list of available RDF Serialization Formats, which includes:

NTriples
Turtle
N3
RDF/XML
JSON::LD

and more

Serializing

Add a predicate to the attributes and relations you wish to serialize.

It's recommended to reuse existing vocabularies provided by the rdf gem and the rdf-vocab gem, and add your own vocab for missing predicates. One way to be able to access the different vocabs throughout your application is by defining a module:

require 'rdf'
require "rdf/vocab"

module NS
  SCHEMA = RDF::Vocab::SCHEMA
  MY_VOCAB = RDF::Vocabulary.new('http://example.com/')
end

Now add the predicates to your serializers.

Old:

class PostSerializer
  include JSONAPI::Serializer
  attributes :title, :body
  belongs_to :author
  has_many :comments
end

New:

class PostSerializer
  include RDF::Serializers::ObjectSerializer
  attribute :title, predicate: NS::SCHEMA[:name]
  attribute :body, predicate: NS::SCHEMA[:text]
  belongs_to :author, predicate: NS::MY_VOCAB[:author]
  has_many :comments, predicate: NS::MY_VOCAB[:comments]
end

For RDF serialization, you are required to add an iri method to your model, which must return a RDF::Resource. For example:

  def iri
    RDF::URI(Rails.application.routes.url_helpers.comment_url(object))
  end

In contrast to the JSON API serializer, this rdf serializers don't automatically serialize the type and id of your model. It's recommended to add attribute :type, predicate: RDF[:type] and a method defining the type to your serializers to fix this.

Custom statements per model

You can add custom statements to the serialization of a model in the serializer, for example:

class PostSerializer
  include RDF::Serializers::ObjectSerializer
  statements :my_custom_statements
  
  def my_custom_statements
    [RDF::Statement.new(RDF::URI('https://example.com'), NS::MY_VOCAB[:fooBar], 1)]
  end
end

Meta statements

You can add additional statements to the serialization in the controller, for example:

render nt: model, meta: [RDF::Statement.new(RDF::URI('https://example.com'), NS::MY_VOCAB[:fooBar], 1)]

Contributing

The usual stuff. Open an issue to discuss a change, open pull requests directly for bugfixes and refactors.