How can I parse OPTIONAL statements from a SPARQL query using Sesame?

Question

I'm playing around with Sesame's queryparser-sparql lib, but I can't seem to get OPTIONAL statements from a parsed query.

Given the query:

PREFIX ex: <http://example.com/#>
SELECT * WHERE {
  ?s a ex:Foo .
  OPTIONAL { ?s ex:someProperty ?property }
} LIMIT 10

Parsing it with the following code (using Sesame 2.7.2):

SPARQLParserFactory factory = new SPARQLParserFactory();
QueryParser parser = factory.getParser();
ParsedQuery parsedQuery = parser.parseQuery(sparqlQuery, null);

StatementPatternCollector collector = new StatementPatternCollector();
TupleExpr tupleExpr = parsedQuery.getTupleExpr();
tupleExpr.visit(collector);

for (StatementPattern pattern : collector.getStatementPatterns()) {
    System.out.println(pattern);
}

printing parsedQuery gives:

Slice ( limit=10 )
   Projection
      ProjectionElemList
         ProjectionElem "s"
         ProjectionElem "property"
      LeftJoin
         StatementPattern
            Var (name=s)
            Var (name=-const-1, value=http://www.w3.org/1999/02/22-rdf-syntax-ns#type, anonymous)
            Var (name=-const-2, value=http://example.com/#Foo, anonymous)
         StatementPattern
            Var (name=s)
            Var (name=-const-3, value=http://example.com/#someProperty, anonymous)
            Var (name=property)

printing each pattern gives:

StatementPattern
   Var (name=s)
   Var (name=-const-1, value=http://www.w3.org/1999/02/22-rdf-syntax-ns#type, anonymous)
   Var (name=-const-2, value=http://example.com/#Foo, anonymous)

StatementPattern
   Var (name=s)
   Var (name=-const-3, value=http://example.com/#someProperty, anonymous)
   Var (name=property)

How can I get information from a StatementPattern on whether or not it is OPTIONAL?

Answer 1

The only way to figure that out is to check whether it occurs as (part of) a right-hand argument of a LeftJoin. A relatively simply way to figure that out is to implement a QueryModelVisitor that sets a flag of some sort whenever it encounters a left join and descends down its right-hand side argument.

Alternatively, you can propagate back up in the query model from the StatementPattern via getParent and inspect the tree like that - this may be more difficult though as the LeftJoin may not necessarily be the immediate parent of the SP.