Module: JSON::LD::FromRDF

Includes:
Utils
Included in:
API
Defined in:
lib/json/ld/from_rdf.rb

Instance Method Summary collapse

Methods included from Utils

#add_value, #as_resource, #blank_node?, #compare_values, #graph?, #has_property, #has_value, #index?, #list?, #node?, #node_or_ref?, #node_reference?, #simple_graph?, #value?

Instance Method Details

#from_statements(input, useRdfType: false, useNativeTypes: false) ⇒ Array<Hash>

Generate a JSON-LD array representation from an array of ‘RDF::Statement`. Representation is in expanded form

Parameters:

  • input (Array<RDF::Statement>, RDF::Enumerable)
  • useRdfType (Boolean) (defaults to: false) —

    (false) If set to ‘true`, the JSON-LD processor will treat `rdf:type` like a normal property instead of using `@type`.

  • useNativeTypes (Boolean) (defaults to: false) —

    (false) use native representations

Returns:

  • (Array<Hash>) —

    the JSON-LD document in normalized form



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# File 'lib/json/ld/from_rdf.rb', line 18

def from_statements(input, useRdfType: false, useNativeTypes: false)
  default_graph = {}
  graph_map = {'@default' => default_graph}
  node_usages_map = {}

  value = nil
  ec = Context.new

  # Create a map for node to object representation

  # For each triple in input
  input.each do |statement|
    #log_debug("statement") { statement.to_nquads.chomp}

    name = statement.graph_name ? ec.expand_iri(statement.graph_name).to_s : '@default'
    
    # Create a graph entry as needed
    node_map = graph_map[name] ||= {}
    default_graph[name] ||= {'@id' => name} unless name == '@default'
    
    subject = ec.expand_iri(statement.subject).to_s
    node = node_map[subject] ||= {'@id' => subject}

    # If object is an IRI or blank node identifier, and node map does not have an object member, create one and initialize its value to a new JSON object consisting of a single member @id whose value is set to object.
    node_map[statement.object.to_s] ||= {'@id' => statement.object.to_s} unless
      statement.object.literal?

    # If predicate equals rdf:type, and object is an IRI or blank node identifier, append object to the value of the @type member of node. If no such member exists, create one and initialize it to an array whose only item is object. Finally, continue to the next RDF triple.
    if statement.predicate == RDF.type && statement.object.resource? && !useRdfType
      merge_value(node, '@type', statement.object.to_s)
      next
    end

    # Set value to the result of using the RDF to Object Conversion algorithm, passing object and use native types.
    value = ec.expand_value(nil, statement.object, useNativeTypes: useNativeTypes, log_depth: @options[:log_depth])

    merge_value(node, statement.predicate.to_s, value)

    # If object is a blank node identifier or IRI, it might represent the a list node:
    if statement.object.resource?
      # Append a new JSON object consisting of three members, node, property, and value to the usages array. The node member is set to a reference to node, property to predicate, and value to a reference to value.
      merge_value(node_map[statement.object.to_s], :usages, {
        node:     node,
        property: statement.predicate.to_s,
        value:    value
      })

      (node_usages_map[statement.object.to_s] ||= []) << node['@id']
    end
  end

  # For each name and graph object in graph map:
  graph_map.each do |name, graph_object|
    next unless nil_var = graph_object[RDF.nil.to_s]

    # For each item usage in the usages member of nil, perform the following steps:
    nil_var.fetch(:usages, []).each do |usage|
      node, property, head = usage[:node], usage[:property], usage[:value]
      list, list_nodes = [], []

      # If property equals rdf:rest, the value associated to the usages member of node has exactly 1 entry, node has a rdf:first and rdf:rest property, both of which have as value an array consisting of a single element, and node has no other members apart from an optional @type member whose value is an array with a single item equal to rdf:List, node represents a well-formed list node. Continue with the following steps:
      #log_debug("list element?") {node.to_json(JSON_STATE) rescue 'malformed json'}
      while property == RDF.rest.to_s &&
          Array(node_usages_map[node['@id']]).uniq.length == 1 &&
          blank_node?(node) &&
          node.keys.none? {|k| !["@id", '@type', :usages, RDF.first.to_s, RDF.rest.to_s].include?(k)} &&
          Array(node[:usages]).length == 1 &&
          (f = node[RDF.first.to_s]).is_a?(Array) && f.length == 1 &&
          (r = node[RDF.rest.to_s]).is_a?(Array) && r.length == 1 &&
          ((t = node['@type']).nil? || t == [RDF.List.to_s])
        list << Array(node[RDF.first.to_s]).first
        list_nodes << node['@id']
        node_usage = Array(node[:usages]).first
        node, property, head = node_usage[:node], node_usage[:property], node_usage[:value]
      end

      # If property equals rdf:first, i.e., the detected list is nested inside another list
      if property == RDF.first.to_s
        # and the value of the @id of node equals rdf:nil, i.e., the detected list is empty, continue with the next usage item. The rdf:nil node cannot be converted to a list object as it would result in a list of lists, which isn't supported.
        next if node['@id'] == RDF.nil.to_s

        # Otherwise, the list consists of at least one item. We preserve the head node and transform the rest of the linked list to a list object
        head_id = head['@id']
        head = graph_object[head_id]
        head = Array(head[RDF.rest.to_s]).first
        list.pop; list_nodes.pop
      end

      head.delete('@id')
      head['@list'] = list.reverse
      list_nodes.each {|node_id| graph_object.delete(node_id)}
    end
  end

  result = []
  default_graph.keys.sort.each do |subject|
    node = default_graph[subject]
    if graph_map.has_key?(subject)
      node['@graph'] = []
      graph_map[subject].keys.sort.each do |s|
        n = graph_map[subject][s]
        n.delete(:usages)
        node['@graph'] << n unless node_reference?(n)
      end
    end
    node.delete(:usages)
    result << node unless node_reference?(node)
  end
  #log_debug("fromRdf") {result.to_json(JSON_STATE) rescue 'malformed json'}
  result
end