Re: [dotNetRDF-bugs] UriNodes in predicate position only don't appear in Graph.Nodes [SEC=UNCLASSIFIED]

[Rob V. <rv...@do...>]

Christopher

This is unintuitive although by design, the Nodes collection only returns
things that are in the Subject/Object position in a triple and not things
that are predicates

This question and my answer at
http://stackoverflow.com/questions/17967686/retrieving-specific-rdf-graph-tr
iples-based-on-predicate-nodes discusses the thinking behind this and the
eventual planned changes around this feature

Rob

From:  "Penny, Christopher" <Chr...@ds...>
Reply-To:  dotNetRDF Bug Report tracking and resolution
<dot...@li...>
Date:  Wednesday, 29 January 2014 20:28
To:  "dot...@li..."
<dot...@li...>
Subject:  [dotNetRDF-bugs] UriNodes in predicate position only don't appear
in Graph.Nodes [SEC=UNCLASSIFIED]

> UNCLASSIFIED
> 
> Hello again all,
>  
> I believe I¹ve stumbled across another (possible) bug. UriNodes which only
> appear in the predicate position do not seem to find their way into
> Graph.Nodes.UriNodes(), Graph.Nodes or become returnable via getUriNode. The
> code below demonstrates the problem I¹m having.
>  
>        privatevoid getUriNodeProblemExample()
>         {
>            //Create graph and add namespace + nodes
>            Graph graph = newGraph();
>             graph.NamespaceMap.AddNamespace("test",
> newUri("http://test.com/test#"));
>            IUriNode subjectNode = graph.CreateUriNode("test:subject");
>            IUriNode predicateNode = graph.CreateUriNode("test:predicate");
>            IUriNode objectNode = graph.CreateUriNode("test:object");
>  
>            //The UriNode is null
>            IUriNode uriNode = graph.GetUriNode("test:subject");
>            if(uriNode == null)
>             {
>                 System.Console.WriteLine("Urinode is null");
>             }
>            else
>             {
>                 System.Console.WriteLine("Returned IUriNode is: " + uriNode);
>             }
>  
>             //Source indicates that GetUriNode iterates over
> graph.Nodes.UriNodes()
>             System.Console.WriteLine("Printing URINodes before first
> triple:");
>            foreach (IUriNode node in graph.Nodes.UriNodes())
>             {
>                 System.Console.WriteLine(">Node in graph.Nodes.UriNodes() is:
> " +node.ToString());
>             }
>  
>            //Iterate over all nodes
>             System.Console.WriteLine("Printing ALL Nodes before first
> triple:");
>            foreach (INode node in graph.Nodes)
>             {
>                 System.Console.WriteLine(">Node: "+node.ToString()+" has type:
> "+node.NodeType);
>             }
>  
>            //Adding a triple seems necessary to get nodes to appear in the
> graph. However, only the subject and object nodes appear after the triple is
> created.
>             graph.Assert(newTriple(subjectNode,predicateNode,objectNode));
>  
>            //Source indicates that GetUriNode iterates over
> graph.Nodes.UriNodes()
>             System.Console.WriteLine("Printing URINodes after first triple:");
>            foreach (IUriNode node in graph.Nodes.UriNodes())
>             {
>                 System.Console.WriteLine(">Node in graph.Nodes.UriNodes() is:
> " + node.ToString());
>             }
>  
>            //Iterate over all nodes
>             System.Console.WriteLine("Printing All nodes after first
> triple:");
>            foreach (INode node in graph.Nodes)
>             {
>                 System.Console.WriteLine(">Node: " + node.ToString() + " has
> type: " + node.NodeType);
>             }
>  
>            //Only the subject and object nodes were found. Lets switch the
> order. All nodes get printed this time.
>             graph.Assert(newTriple(predicateNode, subjectNode, objectNode));
>  
>            //Source indicates that GetUriNode iterates over
> graph.Nodes.UriNodes()
>             System.Console.WriteLine("Printing URINodes after second
> triple:");
>            foreach (IUriNode node in graph.Nodes.UriNodes())
>             {
>                 System.Console.WriteLine(">Node in graph.Nodes.UriNodes() is:
> " + node.ToString());
>             }
>  
>            //Iterate over all nodes
>             System.Console.WriteLine("Printing All nodes after second
> triple:");
>            foreach (INode node in graph.Nodes)
>             {
>                 System.Console.WriteLine(">Node: " + node.ToString() + " has
> type: " + node.NodeType);
>             }
>  
>             System.Console.WriteLine("End of function");
>         }
> 
> I get the following output for the above code:
> Urinode is null
> Printing URINodes before first triple:
> Printing ALL Nodes before first triple:
> Printing URINodes after first triple:
>> >Node in graph.Nodes.UriNodes() is: http://test.com/test#subject
>> >Node in graph.Nodes.UriNodes() is: http://test.com/test#object
> Printing All nodes after first triple:
>> >Node: http://test.com/test#subject has type: Uri
>> >Node: http://test.com/test#object has type: Uri
> Printing URINodes after second triple:
>> >Node in graph.Nodes.UriNodes() is: http://test.com/test#subject
>> >Node in graph.Nodes.UriNodes() is: http://test.com/test#predicate
>> >Node in graph.Nodes.UriNodes() is: http://test.com/test#object
> Printing All nodes after second triple:
>> >Node: http://test.com/test#subject has type: Uri
>> >Node: http://test.com/test#predicate has type: Uri
>> >Node: http://test.com/test#object has type: Uri
> End of function
>  
> I poked around a bit more in the source. Might the problem be in BaseGraph.cs:
>        public virtual IEnumerable<INode> Nodes
>         {
>            get
>             {
>                return (from t in this._triples
>                        select t.Subject).Concat(from t in this._triples
>                                                 select t.Object).Distinct();
>             }
>         }
> Needing to also .Concat t.Predicate?
> 
> Unless the reasoning is that a graph is composed of subject/object nodes with
> predicate Œlinks¹. However, it would be nice to retrieve an IUriNode for a
> predicate from a graph for use with
> Graph.GetTriplesWithSubjectPredicate(INode, INode). To get the rdf:type of the
> subject, for example.
>  
> Let me know if more information is required.
>  
> Regards, Chris.
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