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 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:

Instance Method Summary collapse

Instance Method Details

#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.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 145

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

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 238

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.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 215

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.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 193

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.

Overloads:

Returns:

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 171

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.

Overloads:

  • #first_value ⇒ Object

    Returns:

    • (Object)
  • #first_value(pattern) ⇒ Object

    Parameters:

    Returns:

    • (Object)

Returns:

  • (Object)

Since:

  • 0.1.9



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# File 'lib/rdf/mixin/queryable.rb', line 261

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

#query(pattern) {|statement| ... } ⇒ Enumerator

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:

  • #query_pattern


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# File 'lib/rdf/mixin/queryable.rb', line 31

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