Class: RDF2JSON::Converter

Inherits:
Object
  • Object
show all
Defined in:
lib/rdf2json/rdf2json.rb

Overview

Class that takes an input file (RDF N-Triples/N-Quads) and appends JSON/JSON-LD to a possible pre-existing output file. A namespace and prefix can be given that handle ‘–namespace` and `–prefix` parameters in conjunction with the `–minimize` parameter.

Instance Method Summary collapse

Constructor Details

#initialize(input_filename, output_filename, input_format, output_format, namespace, prefix) ⇒ Converter

Initializes a new converter instance.

input_filename

path/filename of the input file in RDF N-Triples/N-Quads

output_filename

path/filename of the output file to which JSON/JSON-LD is being appended

input_format

format of the input file (:ntriples or :nquads)

output_format

format of the output (:json or jsonld)

namespace

a possible namespace for replacing “@id” keys (may be nil)

prefix

a possible prefix for shortening keys (may be nil)



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# File 'lib/rdf2json/rdf2json.rb', line 146

def initialize(input_filename, output_filename, input_format, output_format, namespace, prefix)
  @input_file = File.open(input_filename, 'r')
  @output_file = File.open(output_filename, 'a')
  @input_format = input_format
  @output_format = output_format
  @namespace = namespace
  @prefix = prefix
end

Instance Method Details

#convert ⇒ Object

Convert the input file by appending the newly formatted data to the output file.

At the end of the conversion a short statistic is output. It tells the number of lines read from the input file, the number of errors in the N-Triples/N-Quads file, the number of JSON/JSON-LD documents appended to the output file (equiv. to number of lines appended).



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# File 'lib/rdf2json/rdf2json.rb', line 161

def convert
  no_of_lines = 0
  no_of_statements = 0
  read_errors = 0
  last_subject = nil
  subject_block = ''

  @input_file.each_line { |line|
    no_of_lines += 1
    line.chomp!

    subject = "#{line.sub(/>.*/, '')}>"

    if subject == last_subject then
      subject_block << line
    else
      stats = write_graph(subject_block)
      no_of_statements += stats[:no_of_statements]
      read_errors += stats[:read_errors]
      subject_block = ''
    end

    last_subject = subject
  }

  stats = write_graph(subject_block)
  no_of_statements += stats[:no_of_statements]
  read_errors += stats[:read_errors]

  puts "Total number of lines read                   : #{no_of_lines}"
  puts "Statement read errors (N-Quads or N-Triples) : #{read_errors}"
  puts "JSON/JSON-LD documents output                : #{no_of_statements}"
end

#minify(jsonld_hash) ⇒ Object

Minimize a JSON-LD hash to JSON.

jsonld_hash

a JSON-LD hash that should be rewritten to plain JSON



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# File 'lib/rdf2json/rdf2json.rb', line 198

def minify(jsonld_hash)
  jsonld_hash.keys.each { |key|
    if key == '@type' then
      jsonld_hash.delete(key)
    elsif @prefix and key.match(@prefix) then
      shortened_key = key.sub(@prefix, '')
      jsonld_hash[shortened_key] = jsonld_hash.delete(key)
      key = shortened_key
    end

    if jsonld_hash[key].instance_of?(Array) then
      jsonld_hash[key].each_index { |index|
        if jsonld_hash[key][index].has_key?('@value') then
          jsonld_hash[key][index] = jsonld_hash[key][index]['@value']
        elsif jsonld_hash[key][index].has_key?('@id') then
          jsonld_hash[key][index] = jsonld_hash[key][index]['@id']
        end
      }
    elsif jsonld_hash[key].instance_of?(Hash) then
      minify(jsonld_hash[key])
    end
  }
end

#write_graph(block) ⇒ Object

Takes a block of RDF statements that share the same subject and creates a JSON/JSON-LD document from them, which is appended to the output file.

block

one or more lines that share the same subject in RDF N-Triples/N-Quads



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# File 'lib/rdf2json/rdf2json.rb', line 226

def write_graph(block)
  return { :read_errors => 0, :no_of_statements => 0 } unless block and not block.empty?

  # Virtuoso output error-handling:
  block.gsub!("\\'", "'")

  read_errors = 0
  no_of_statements = 0
  graph = RDF::Graph.new
  RDF::Reader.for(@input_format).new(block) { |reader|
    begin
      reader.each_statement { |statement|
        no_of_statements += 1
        graph.insert(statement)
      }
    rescue RDF::ReaderError
      read_errors += 1
    end
  }

  JSON::LD::API::fromRdf(graph) { |document|
    document.each{ |entity|
      # Parsed JSON-LD representation:
      entity = JSON.parse(entity.to_json)

      entity[@namespace] = entity.delete('@id') if @namespace
      minify(entity) if @output_format == :json

      @output_file.puts entity.to_json
    }
  }

  return { :read_errors => read_errors, :no_of_statements => no_of_statements }
end