Module: RDF::Queryable

Includes:
Enumerable
Included in:
Dataset, Enumerable::Enumerator, Graph, Enumerator, Transaction
Defined in:
lib/rdf/mixin/queryable.rb,
lib/rdf/mixin/enumerator.rb

Overview

An RDF query mixin.

Classes that include this module should implement a ‘#query_pattern` method that yields RDF statements. Classes may also implement an optimized `#query_execute` method that yields RDF statements.

See Also:

Defined Under Namespace

Classes: Enumerator

Instance Method Summary collapse

Instance Method Details

#enum_for(method = :each, *args) ⇒ Enumerator<RDF::Statement, RDF::Query::Pattern> Also known as: to_enum

See Also:

  • Object#enum_for


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# File 'lib/rdf/mixin/queryable.rb', line 287

def enum_for(method = :each, *args)
  # Ensure that enumerators are, themselves, queryable
  this = self
  Queryable::Enumerator.new do |yielder|
    this.send(method, *args) {|*y| yielder << (y.length > 1 ? y : y.first)}
  end
end

#first ⇒ RDF::Statement #first(pattern) ⇒ RDF::Statement

Queries ‘self` for an RDF statement matching the given `pattern` and returns that statement if found.

Returns ‘nil` if no statements match `pattern`.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 166

def first(pattern = nil)
  if pattern
    query(pattern) do |statement|
      return statement
    end
  elsif respond_to?(:each_statement)
    each_statement do |statement|
      return statement
    end
  else
    return super()
  end
  nil
end

#first_literal ⇒ RDF::Literal #first_literal(pattern) ⇒ RDF::Literal

Queries ‘self` for RDF statements matching the given `pattern` and returns the first found object literal.

Returns ‘nil` if no statements match `pattern` or if none of the found statements have a literal as their object term.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 257

def first_literal(pattern = nil)
  __send__(*(pattern ? [:query, pattern] : [:each])) do |statement|
    return statement.object if statement.object.is_a?(RDF::Literal)
  end
  return nil
end

#first_object ⇒ RDF::Term #first_object(pattern) ⇒ RDF::Term

Queries ‘self` for an RDF statement matching the given `pattern` and returns the statement’s object term.

Returns ‘nil` if no statements match `pattern`.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 234

def first_object(pattern = nil)
  __send__(*(pattern ? [:query, pattern] : [:each])) do |statement|
    return statement.object
  end
  return nil
end

#first_predicate ⇒ RDF::URI #first_predicate(pattern) ⇒ RDF::URI

Queries ‘self` for an RDF statement matching the given `pattern` and returns the statement’s predicate term.

Returns ‘nil` if no statements match `pattern`.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 214

def first_predicate(pattern = nil)
  __send__(*(pattern ? [:query, pattern] : [:each])) do |statement|
    return statement.predicate
  end
  return nil
end

#first_subject ⇒ RDF::Resource #first_subject(pattern) ⇒ RDF::Resource

Queries ‘self` for an RDF statement matching the given `pattern` and returns the statement’s subject term.

Returns ‘nil` if no statements match `pattern`.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 194

def first_subject(pattern = nil)
  __send__(*(pattern ? [:query, pattern] : [:each])) do |statement|
    return statement.subject
  end
  return nil
end

#first_value ⇒ Object #first_value(pattern) ⇒ Object

Queries ‘self` for RDF statements matching the given `pattern` and returns the value of the first found object literal.

Returns ‘nil` if no statements match `pattern` or if none of the found statements have a literal as their object term.

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 278

def first_value(pattern = nil)
  (literal = first_literal(pattern)) ? literal.value : nil
end

#query(pattern, options = {}) {|statement| ... } ⇒ Enumerator<RDF::Statement>, ...

Note:

Since 2.0, this may return an Enumerable or an Enumerator in addition to Solutions

Queries ‘self` for RDF statements matching the given `pattern`.

This method delegates to the protected #query_pattern method for the actual lower-level query pattern matching implementation.

Examples:

Querying for statements having a given predicate

queryable.query([nil, RDF::Vocab::DOAP.developer, nil])
queryable.query(predicate: RDF::Vocab::DOAP.developer) do |statement|
  puts statement.inspect
end

Querying for solutions from a BGP

query = RDF::Query.new {pattern [:s, :p, :o]}
queryable.query(query) do |solution|
  puts solution.inspect
end

Yields:

  • (statement) —

    each matching statement

Yield Parameters:

Yield Returns:

  • (void) —

    ignored

Raises:

  • (TypeError)

See Also:

  • #query_pattern


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# File 'lib/rdf/mixin/queryable.rb', line 44

def query(pattern, options = {}, &block)
  raise TypeError, "#{self} is not readable" if respond_to?(:readable?) && !readable?

  case pattern
    # A basic graph pattern (BGP) query:
    when Query
      solutions = RDF::Query::Solutions.new
      block = lambda {|solution| solutions << solution} unless block_given?
      before_query(pattern) if respond_to?(:before_query)
      query_execute(pattern, options, &block)
      after_query(pattern) if respond_to?(:after_query)
      # Returns the solutions, not an enumerator
      solutions
 
    # A simple triple/quad pattern query:
    else
      pattern = Query::Pattern.from(pattern)
      before_query(pattern) if respond_to?(:before_query)
      enum = case
        # Blank triple/quad patterns are equivalent to iterating over
        # every statement, so as a minor optimization we'll just do that
        # directly instead of bothering with `#query_pattern`:
        when pattern.blank?
          if block_given?
            each(&block)
          else
            to_a.extend(Queryable)
          end

        # Constant triple/quad patterns are equivalent to looking up a
        # particular statement, so as a minor optimization we'll just do
        # that directly instead of bothering with `#query_pattern`:
        when pattern.constant?
          statement = Statement.from(pattern)
          if include?(statement)
            if block_given?
              yield statement
            else
              [statement]
            end
          end

        # Otherwise, we delegate to `#query_pattern`:
        else # pattern.variable?
          query_pattern(pattern, options, &block)
      end
      after_query(pattern) if respond_to?(:after_query)
      enum
  end
end