class SPARQL::Algebra::Operator::Construct

The SPARQL GraphPattern `construct` operator.

The CONSTRUCT query form returns a single RDF graph specified by a graph template. The result is an RDF graph formed by taking each query solution in the solution sequence, substituting for the variables in the graph template, and combining the triples into a single RDF graph by set union.

@example

(prefix ((rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>)
         (foaf: <http://xmlns.com/foaf/0.1/>))
  (construct ((triple ?s ?p ?o))
    (project (?s ?p ?o)
      (bgp (triple ?s ?p ?o)))))

@see www.w3.org/TR/sparql11-query/#construct

Constants

NAME

Public Instance Methods

execute(queryable, **options, &block) click to toggle source

Executes this query on the given {RDF::Queryable} object. Binds variables to the array of patterns in the first operand and returns the resulting RDF::Graph object

If any such instantiation produces a triple containing an unbound variable or an illegal RDF construct, such as a literal in subject or predicate position, then that triple is not included in the output RDF graph. The graph template can contain triples with no variables (known as ground or explicit triples), and these also appear in the output RDF graph returned by the CONSTRUCT query form.

@param [RDF::Queryable] queryable

the graph or repository to query

@param [Hash{Symbol => Object}] options

any additional keyword options

@yield [statement]

each matching statement

@yieldparam [RDF::Statement] solution @yieldreturn [void] ignored @return [RDF::Queryable]

A Queryable with constructed triples

@see www.w3.org/TR/sparql11-query/#construct

# File lib/sparql/algebra/operator/construct.rb, line 38
def execute(queryable, **options, &block)
  debug(options) {"Construct #{operands.first}, #{options.inspect}"}
  graph = RDF::Graph.new
  patterns = operands.first
  query = operands.last

  queryable.query(query, depth: options[:depth].to_i + 1, **options).each do |solution|
    debug(options) {"(construct apply) #{solution.inspect} to BGP"}
    
    # Create a mapping from BNodes within the pattern list to newly constructed BNodes
    nodes = {}
    patterns.each do |pattern|
      terms = {}
      [:subject, :predicate, :object].each do |r|
        terms[r] = case o = pattern.send(r)
        when RDF::Node            then nodes[o] ||= RDF::Node.new
        when RDF::Query::Variable then solution[o]
        when RDF::Query::Pattern  then RDF::Statement.from(o.dup.bind(solution))
        else                           o
        end
      end

      statement = RDF::Statement.from(terms)

      # Sanity checking on statement
      if statement.subject.nil? || statement.predicate.nil? || statement.object.nil? ||
         statement.subject.literal? || statement.predicate.literal?
        debug(options) {"(construct skip) #{statement.inspect}"}
        next
      end

      debug(options) {"(construct add) #{statement.inspect}"}
      graph << statement
    end
  end

  debug(options) {"=>\n#{graph.dump(:ttl, standard_prefixes: true)}"}
  graph.each(&block) if block_given?
  graph
end
query_yields_statements?() click to toggle source

Query results statements (e.g., CONSTRUCT, DESCRIBE, CREATE) @return [Boolean]

# File lib/sparql/algebra/operator/construct.rb, line 81
def query_yields_statements?
  true
end