rdf - 基于谓词节点检索特定 RDF 图三元组

Question

我正在使用dotnetrdf发送 SparqlCONSTRUCT查询并评估结果IGraph。

首先，我试图找到代表给定类型个体的所有 Uri 节点，例如，

http://www.example.com/InterestingThing

所以，我想使用GetTriplesWithPredicateObject方法http://www.w3.org/1999/02/22-rdf-syntax-ns#type，作为谓词和对象传递http://www.example.com/InterestingThing。该方法需要两个对象实现INode接口，所以我试图用图形的GetUriNode方法找到相应的节点。

问题：未找到谓词。

在调试器中查看IGraph对象时，似乎GetUriNode只能找到图形Nodes枚举中包含的内容 - 并且仅包含已用作主题或对象的 Uri 节点，但没有用作谓词。

现在，谓词 Uri 节点显然在那里——我可以在探索图形Triples对象时看到它们。它们出现在可以直接从该对象检索的Triple实例中，也可以在同一对象的PredicateNodes属性中找到。

有趣的是，IGraph诸如GetTriplesWithObjector的某些查询方法GetTriplesWithPredicate具有仅接受 aUri而不是INode- 但GetTriplesWithPredicateObject不接受的重载。

现在，各种可能的解决方法很明显：

• 使用GetTriplesWithPredicate仅需要一个Uri实例的重载，然后手动过滤生成的三元组。
• 手动搜索所有三元组以获得所需的三元组。
• 手动搜索列表PredicateNodes并找到搜索到 Uris 的列表。

不过，所有这些似乎都不必要地繁琐——因为现有方法乍一看表明应该能够简单地使用来GetUriNode检索两个节点，然后将它们传递给现成的查询方法之一，例如GetTriplesWithObjectPredicate.

因此，我的问题是：

我是否遗漏了任何东西，无论是在 dotnetrdf 库中还是在概念上，在 RDF 中，或者这种行为是否存在设计给开发人员带来的障碍？

这是一个重现问题的 C# 测试用例（查询 DBpedia）：

using System;
using System.Collections.Generic;
using System.Linq;

using VDS.RDF;
using VDS.RDF.Query;

namespace Test
{
    class Program
    {
        public static void Main(string[] args)
        {
            // Querying
            // All entries to analyze are declared to belong to a temporary type, and
            // two properties are mapped to temporary property identifiers.

            Console.WriteLine();

            var endpoint = new SparqlRemoteEndpoint(new Uri("http://dbpedia.org/sparql"));

            string query = "PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>\n"
                + "PREFIX ex: <http://www.example.com/>\n"
                + "\n"
                + "CONSTRUCT {\n"
                + "  ?r a ex:record.\n"
                + "  ?r ex:prop1 ?v1.\n"
                + "  ?r ex:prop2 ?v2.\n"
                + "}\n"
                + "WHERE {\n"
                + "  {\n"
                + "    SELECT ?r\n"
                + "    WHERE {\n"
                + "      ?r a dbpedia-owl:Work.\n"
                + "      FILTER(EXISTS { ?r dbpedia-owl:originalLanguage [] } && EXISTS { ?r dbpedia-owl:author/dbpedia-owl:birthPlace [] }).\n"
                + "    } LIMIT 5\n"
                + "  }\n"
                + "  ?r dbpedia-owl:originalLanguage ?v1.\n"
                + "  ?r dbpedia-owl:author/dbpedia-owl:birthPlace ?v2.\n"
                + "}";

            Console.WriteLine(query);
            Console.WriteLine();

            IGraph graph = endpoint.QueryWithResultGraph(query);

            PrintNodes("IGraph.Nodes", graph.Nodes);

            Console.WriteLine("All triples from IGraph.Triples:");
            foreach (var triple in graph.Triples) {
                Console.WriteLine("- S: " + NodeToString(triple.Subject));
                Console.WriteLine("  P: " + NodeToString(triple.Predicate));
                Console.WriteLine("  O: " + NodeToString(triple.Object));
            }
            Console.WriteLine();

            PrintNodes("IGraph.Triples.SubjectNodes", graph.Triples.SubjectNodes);
            PrintNodes("IGraph.Triples.PredicateNodes", graph.Triples.PredicateNodes);
            PrintNodes("IGraph.Triples.ObjectNodes", graph.Triples.ObjectNodes);

            // Graph Analysis
            // The following code tries to retrieve exactly the items of the temporary
            // type "record".

            var typeNode = TryFindUriNode(graph, new Uri(NamespaceMapper.RDF + "type"));
            var recordNode = TryFindUriNode(graph, new Uri("http://www.example.com/record"));

            Console.WriteLine();
            TryFindAllNodes("subjects", graph, graph.Triples.SubjectNodes.OfType<IUriNode>().Select(node => node.Uri));
            TryFindAllNodes("predicates", graph, graph.Triples.PredicateNodes.OfType<IUriNode>().Select(node => node.Uri));
            TryFindAllNodes("objects", graph, graph.Triples.ObjectNodes.OfType<IUriNode>().Select(node => node.Uri));
            Console.WriteLine();

            var createdTypeNode = graph.CreateUriNode(new Uri(NamespaceMapper.RDF + "type"));
            var createdRecordNode = graph.CreateUriNode(new Uri("http://www.example.com/record"));

            if ((typeNode != null) && (recordNode != null)) {
                Console.WriteLine("{0} triple(s) with found predicate and found object.",
                                  graph.GetTriplesWithPredicateObject(typeNode, recordNode).Count());
            }
            if (typeNode != null) {
                Console.WriteLine("{0} triple(s) with found predicate and created object.",
                                  graph.GetTriplesWithPredicateObject(typeNode, createdRecordNode).Count());
            }
            if (recordNode != null) {
                Console.WriteLine("{0} triple(s) with created predicate and found object.",
                                  graph.GetTriplesWithPredicateObject(createdTypeNode, recordNode).Count());
            }
            Console.WriteLine("{0} triple(s) with created predicate and created object.",
                              graph.GetTriplesWithPredicateObject(createdTypeNode, createdRecordNode).Count());
        }

        private static string NodeToString(INode node)
        {
            return string.Format("{0} ({1})", node, node.GetType().Name);
        }

        private static void PrintNodes(string title, IEnumerable<INode> nodes)
        {
            Console.WriteLine(title + ":");
            foreach (var node in nodes) {
                Console.WriteLine("- " + NodeToString(node));
            }
            Console.WriteLine();
        }

        private static INode TryFindUriNode(IGraph graph, Uri uri)
        {
            var result = graph.GetUriNode(uri);
            if (result == null) {
                Console.WriteLine(uri.ToString() + " was NOT found by IGraph.GetUriNode.");
            } else {
                Console.WriteLine(uri.ToString() + " WAS found by IGraph.GetUriNode.");
            }
            return result;
        }

        private static void TryFindAllNodes(string title, IGraph graph, IEnumerable<Uri> uris)
        {
            Console.WriteLine("Trying to find all " + title + ":");
            foreach (Uri uri in uris) {
                TryFindUriNode(graph, uri);
            }
        }
    }
}

这是示例程序的输出：

PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
PREFIX ex: <http://www.example.com/>

CONSTRUCT {
  ?r a ex:record.
  ?r ex:prop1 ?v1.
  ?r ex:prop2 ?v2.
}
WHERE {
  {
    SELECT ?r
    WHERE {
      ?r a dbpedia-owl:Work.
      FILTER(EXISTS { ?r dbpedia-owl:originalLanguage [] } && EXISTS { ?r dbpedia-owl:author/dbpedia-owl:birthPlace [] }).
    } LIMIT 5
  }
  ?r dbpedia-owl:originalLanguage ?v1.
  ?r dbpedia-owl:author/dbpedia-owl:birthPlace ?v2.
}

IGraph.Nodes:
- http://dbpedia.org/resource/Electra_(Sophocles) (UriNode)
- http://dbpedia.org/resource/Peer_Gynt (UriNode)
- http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
- http://dbpedia.org/resource/The_Field (UriNode)
- http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)
- http://www.example.com/record (UriNode)
- http://dbpedia.org/resource/Ancient_Greek (UriNode)
- http://dbpedia.org/resource/Colonus (UriNode)
- http://dbpedia.org/resource/Norwegian_language (UriNode)
- http://dbpedia.org/resource/Skien (UriNode)
- http://dbpedia.org/resource/Norway (UriNode)
- http://dbpedia.org/resource/Czech_language (UriNode)
- http://dbpedia.org/resource/Kingdom_of_Bohemia (UriNode)
- http://dbpedia.org/resource/Bohemia (UriNode)
- http://dbpedia.org/resource/Mal%C3%A9_Svato%C5%88ovice (UriNode)
- http://dbpedia.org/resource/Austria-Hungary (UriNode)
- http://dbpedia.org/resource/English_language (UriNode)
- http://dbpedia.org/resource/Irish_Free_State (UriNode)
- http://dbpedia.org/resource/Listowel (UriNode)
- http://dbpedia.org/resource/County_Kerry (UriNode)

All triples from IGraph.Triples:
- S: http://dbpedia.org/resource/Electra_(Sophocles) (UriNode)
  P: http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
  O: http://www.example.com/record (UriNode)
- S: http://dbpedia.org/resource/Electra_(Sophocles) (UriNode)
  P: http://www.example.com/prop1 (UriNode)
  O: http://dbpedia.org/resource/Ancient_Greek (UriNode)
- S: http://dbpedia.org/resource/Electra_(Sophocles) (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Colonus (UriNode)
- S: http://dbpedia.org/resource/Peer_Gynt (UriNode)
  P: http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
  O: http://www.example.com/record (UriNode)
- S: http://dbpedia.org/resource/Peer_Gynt (UriNode)
  P: http://www.example.com/prop1 (UriNode)
  O: http://dbpedia.org/resource/Norwegian_language (UriNode)
- S: http://dbpedia.org/resource/Peer_Gynt (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Skien (UriNode)
- S: http://dbpedia.org/resource/Peer_Gynt (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Norway (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
  O: http://www.example.com/record (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.example.com/prop1 (UriNode)
  O: http://dbpedia.org/resource/Czech_language (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Kingdom_of_Bohemia (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Bohemia (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Mal%C3%A9_Svato%C5%88ovice (UriNode)
- S: http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Austria-Hungary (UriNode)
- S: http://dbpedia.org/resource/The_Field (UriNode)
  P: http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
  O: http://www.example.com/record (UriNode)
- S: http://dbpedia.org/resource/The_Field (UriNode)
  P: http://www.example.com/prop1 (UriNode)
  O: http://dbpedia.org/resource/English_language (UriNode)
- S: http://dbpedia.org/resource/The_Field (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Irish_Free_State (UriNode)
- S: http://dbpedia.org/resource/The_Field (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Listowel (UriNode)
- S: http://dbpedia.org/resource/The_Field (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/County_Kerry (UriNode)
- S: http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)
  P: http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
  O: http://www.example.com/record (UriNode)
- S: http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)
  P: http://www.example.com/prop1 (UriNode)
  O: http://dbpedia.org/resource/Norwegian_language (UriNode)
- S: http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Skien (UriNode)
- S: http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)
  P: http://www.example.com/prop2 (UriNode)
  O: http://dbpedia.org/resource/Norway (UriNode)

IGraph.Triples.SubjectNodes:
- http://dbpedia.org/resource/Electra_(Sophocles) (UriNode)
- http://dbpedia.org/resource/Peer_Gynt (UriNode)
- http://dbpedia.org/resource/Pictures_from_the_Insects'_Life (UriNode)
- http://dbpedia.org/resource/The_Field (UriNode)
- http://dbpedia.org/resource/The_Lady_from_the_Sea (UriNode)

IGraph.Triples.PredicateNodes:
- http://www.w3.org/1999/02/22-rdf-syntax-ns#type (UriNode)
- http://www.example.com/prop1 (UriNode)
- http://www.example.com/prop2 (UriNode)

IGraph.Triples.ObjectNodes:
- http://www.example.com/record (UriNode)
- http://dbpedia.org/resource/Ancient_Greek (UriNode)
- http://dbpedia.org/resource/Colonus (UriNode)
- http://dbpedia.org/resource/Norwegian_language (UriNode)
- http://dbpedia.org/resource/Skien (UriNode)
- http://dbpedia.org/resource/Norway (UriNode)
- http://dbpedia.org/resource/Czech_language (UriNode)
- http://dbpedia.org/resource/Kingdom_of_Bohemia (UriNode)
- http://dbpedia.org/resource/Bohemia (UriNode)
- http://dbpedia.org/resource/Mal%C3%A9_Svato%C5%88ovice (UriNode)
- http://dbpedia.org/resource/Austria-Hungary (UriNode)
- http://dbpedia.org/resource/English_language (UriNode)
- http://dbpedia.org/resource/Irish_Free_State (UriNode)
- http://dbpedia.org/resource/Listowel (UriNode)
- http://dbpedia.org/resource/County_Kerry (UriNode)

Trying to find all subjects:
http://dbpedia.org/resource/Electra_(Sophocles) WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Peer_Gynt WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Pictures_from_the_Insects'_Life WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/The_Field WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/The_Lady_from_the_Sea WAS found by IGraph.GetUriNode.
Trying to find all predicates:
http://www.w3.org/1999/02/22-rdf-syntax-ns#type was NOT found by IGraph.GetUriNode.
http://www.example.com/prop1 was NOT found by IGraph.GetUriNode.
http://www.example.com/prop2 was NOT found by IGraph.GetUriNode.
Trying to find all objects:
http://www.example.com/record WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Ancient_Greek WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Colonus WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Norwegian_language WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Skien WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Norway WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Czech_language WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Kingdom_of_Bohemia WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Bohemia WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Mal‚_Svatonovice WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Austria-Hungary WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/English_language WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Irish_Free_State WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/Listowel WAS found by IGraph.GetUriNode.
http://dbpedia.org/resource/County_Kerry WAS found by IGraph.GetUriNode.

http://www.w3.org/1999/02/22-rdf-syntax-ns#type was NOT found by IGraph.GetUriNode.
http://www.example.com/record WAS found by IGraph.GetUriNode.
5 triple(s) with created predicate and found object.
5 triple(s) with created predicate and created object.

可以看出，IGraph.Nodes枚举不包括谓词 Uris，例如http://www.example.com/prop1or http://www.w3.org/1999/02/22-rdf-syntax-ns#type。同样，输出显示谓词不是由 找到的GetUriNode，而主语和宾语是。

Answer 1

您可以使用INodeFactory （扩展）中的CreateURINodeIGraph来创建rdf:type节点，并具有以下内容：

graph.GetTriplesWithPredicateObject(
  graph.CreateUriNode( UriFactory.createUri( "http://www.w3.org/1999/02/22-rdf-syntax-ns#type" )), ,
  graph.CreateUriNode( UriFactory.createUri( "http://www.example.com/InterestingThing" ))
)

这类似于我在耶拿采取的方法，我会使用类似ResourceFactory.createProperty( http://...ns#type ). （实际上，RDF.type为了方便起见，Jena 有一个常量供我使用，但这是我用于未提前定义为常量的方法。）

Answer 2

这种行为完全是设计使然，只出现在谓词位置的东西被认为是图中的一条边，而不是图中的一个节点。

正如Working with Graphs文档所述：

要选择节点，有一些方法可用于从图中查找节点（如果存在），这些GetXNode()方法中 X 是要检索的节点的类型。请注意，如果给定值作为图中的节点存在，则此方法仅返回一个值，即它出现在此图中的三元组的主题/对象位置。

注意：如果您只想获取 Node 实例以用于其他用途，而不管它是否已存在于图中，您应该使用这些CreateXNode()方法。

也许一个GetEdge()和相应的Edges属性会有用？