Creating a pattern from a property path

Question

Given an RDF::URI and a SPARQL property path (stored as a String), I want to find values that satisfy the following query

SELECT ?value WHERE {
    <URI> <property_path> ?value .
}

I managed to get by using the following snippet, but is feels very hackish:

query = SPARQL.parse(<<~QUERY)       
  SELECT ?value WHERE {              
    <#{uri}> #{property_path} ?value     
  }                                  
QUERY                                
graph.query(query)

Is there a better way to achieve this, using a RDF::Query for example?

Answer 1

From my understanding, you consider string interpolation to be "hackish" because you would like to deal with "things, not strings". This desire is definitely not reprehensible in a Semantic Web related field.

If so, you could construct queries using SPARQL::Algebra.

All snippets below have the same meaning:

SPARQL query (q1)

SELECT ?value WHERE {              
    <http://example.org/green-goblin>
    <http://www.w3.org/2000/01/rdf-schema#label>|<http://xmlns.com/foaf/0.1/name>
    ?value .    
}

SPARQL Algebra expression (q2)

(project
    ?value
    (path
        <http://example.org/green-goblin>
        (alt
            <http://www.w3.org/2000/01/rdf-schema#label>
            <http://xmlns.com/foaf/0.1/name>
        )
        ?value
    )
)    

Ruby code (q3)

Operator::Project.new(
    Variable(:value),
    Operator::Path.new(
        RDF::URI('http://example.org/green-goblin'),
        Operator::Alt.new(
            RDF::URI('http://www.w3.org/2000/01/rdf-schema#label'),
            RDF::URI('http://xmlns.com/foaf/0.1/name')
        ),
    Variable(:value)
    )
)

Compare internal representations or queries results:

require 'rubygems'
require 'sparql'
require 'sparql/algebra'

include SPARQL::Algebra

query1 = SPARQL.parse(q1) 
query2 = SPARQL::Algebra::Expression.parse(q2)
query3 = eval(q3)

puts query1.to_sxp
puts query2.to_sxp
puts query3.to_sxp

query1.execute(queryable) do |result|
  puts result.inspect
end

query2.execute(queryable) do |result|
  puts result.inspect
end

query3.execute(queryable) do |result|
  puts result.inspect
end

The difference is that you do not need string manipulations in the third case.
These "operators" even have URIs (e.g. Operator[:project]).