On 5/23/2013 0:53, [email protected] wrote:
Hey,
I'm working on a SPIN-based query builder. One of the last things I
added is a possibility to specify a Variable or expression to be used
in ORDER BY.
This is working fine, except that the supplied Variable might not be
used in the WHERE pattern, which produces an invalid SPARQL query.
I want to fix this by checking if the supplied Variable is present in
the WHERE pattern, and throw an exception if it's not. The method I'm
using:
protected boolean isWhereVariable(Variable var)
{
Iterator<Element> it = getWhere().getElements().iterator();
while (it.hasNext())
{
Element element = it.next();
RDFNode subject =
element.getRequiredProperty(SP.subject).getObject();
if (subject.canAs(Variable.class) &&
subject.as(Variable.class).getName().equals(var.getName()))
return true;
RDFNode predicate =
element.getRequiredProperty(SP.predicate).getObject();
if (predicate.canAs(Variable.class) &&
predicate.as(Variable.class).getName().equals(var.getName()))
return true;
RDFNode object =
element.getRequiredProperty(SP.object).getObject();
if (object.canAs(Variable.class) &&
object.as(Variable.class).getName().equals(var.getName()))
return true;
}
return false;
}
The problem is that subject.canAs(Variable.class) returns false, even
if stepping through the code I can see, that
subject.as(Variable.class) is "?instance" for example (the variable
I'm expecting).
Any ideas on why the canAs() check doesn't work, or how this could be
implemented in a better way?
canAs() probably relies on an rdf:type triple, but Variables usually
don't have those. Instead use SPINFactory.isVariable(node).
The loop above looks rather fragile though because it only handles BGPs,
i.e. nested blocks would be ignored. This may work in your specific
case, but instead you may want to do a recursive traversal. I believe
the following does something close to what you need (from SPINUtil):
public static Integer containsThis(CommandWithWhere command) {
return new ContainsVarChecker(SPIN._this).checkDepth(command);
}
HTH
Holger
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