Module: RDF::Queryable

Includes:
Enumerable
Included in:
Graph, Repository
Defined in:
lib/rdf/mixin/queryable.rb

Overview

An RDF query mixin.

Classes that include this module must implement an #each method that yields RDF statements.

See Also:

Instance Method Summary (collapse)

Instance Method Details

- (RDF::Statement) first - (RDF::Statement) first(pattern)

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

Returns nil if no statements match pattern.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 144

def first(pattern = nil)
  if pattern
    query(pattern) do |statement|
      return statement
    end
    return nil
  else
    super()
  end
end

- (RDF::Literal) first_literal - (RDF::Literal) first_literal(pattern)

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.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 237

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

- (RDF::Term) first_object - (RDF::Term) first_object(pattern)

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

Returns nil if no statements match pattern.

Overloads:

Returns:

Since:

  • 0.1.9



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

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

- (RDF::URI) first_predicate - (RDF::URI) first_predicate(pattern)

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

Returns nil if no statements match pattern.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 192

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

- (RDF::Resource) first_subject - (RDF::Resource) first_subject(pattern)

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

Returns nil if no statements match pattern.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 170

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

- (Object) first_value - (Object) first_value(pattern)

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.

Overloads:

  • - (Object) first_value

    Returns:

    • (Object)
  • - (Object) first_value(pattern)

    Parameters:

    Returns:

    • (Object)

Returns:

  • (Object)

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 260

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

- (Enumerator) query(pattern) {|statement| ... }

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:

queryable.query([nil, RDF::DOAP.developer, nil])
queryable.query(:predicate => RDF::DOAP.developer)

Parameters:

Yields:

  • (statement) —

    each matching statement

Yield Parameters:

Yield Returns:

  • (void) —

    ignored

Returns:

Raises:

  • (TypeError)

See Also:



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# File 'lib/rdf/mixin/queryable.rb', line 30

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

  case pattern
    # A basic graph pattern (BGP) query:
    when Query
      if block_given?
        before_query(pattern) if respond_to?(:before_query)
        query_execute(pattern, &block)
        after_query(pattern) if respond_to?(:after_query)
      end
      enum_for(:query_execute, pattern)

    # A simple triple/quad pattern query:
    else
      pattern = Query::Pattern.from(pattern)
      before_query(pattern) if block_given? && 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?
          each(&block) if block_given?
          enum_for(:each)

        # 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)
          block.call(statement) if block_given? && include?(statement)
          enum_for(:query, pattern)

        # Otherwise, we delegate to `#query_pattern`:
        else # pattern.variable?
          query_pattern(pattern, &block) if block_given?
          enum_for(:query_pattern, pattern)
      end
      after_query(pattern) if block_given? && respond_to?(:after_query)
      enum.extend(RDF::Queryable, RDF::Enumerable, RDF::Countable)
      def enum.to_a
        super.extend(RDF::Queryable, RDF::Enumerable, RDF::Countable)
      end
      enum
  end
end

- query_execute(query) {|solution| ... } (protected)

This method returns an undefined value.

Queries self using the given basic graph pattern (BGP) query, yielding each matched solution to the given block.

Since RDF.rb 0.3.0, repository implementations can override this method in order to provide for storage-specific optimized graph pattern query execution.

Parameters:

Yields:

  • (solution)

Yield Parameters:

Yield Returns:

  • (void) —

    ignored

See Also:

Since:

  • 0.3.0



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# File 'lib/rdf/mixin/queryable.rb', line 94

def query_execute(query, &block)
  # By default, we let RDF.rb's built-in `RDF::Query#execute` handle BGP
  # query execution by breaking down the query into its constituent
  # triple patterns and invoking `RDF::Query::Pattern#execute` on each
  # pattern.
  query.execute(self).each(&block)
end

- query_pattern(pattern) {|statement| ... } (protected)

This method returns an undefined value.

Queries self for RDF statements matching the given pattern, yielding each matched statement to the given block.

Since RDF.rb 0.2.0, repository implementations should override this method in order to provide for storage-specific optimized triple pattern matching.

Parameters:

Yields:

  • (statement)

Yield Parameters:

Yield Returns:

  • (void) —

    ignored

See Also:

Since:

  • 0.2.0



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# File 'lib/rdf/mixin/queryable.rb', line 120

def query_pattern(pattern, &block)
  # By default, we let Ruby's built-in `Enumerable#grep` handle the
  # matching of statements by iterating over all statements and calling
  # `RDF::Query::Pattern#===` on each statement.
  # @see http://ruby-doc.org/core/classes/Enumerable.html#M003121
  grep(pattern, &block)
end