python - 使用空白节点 (rdflib) 时，Sparql 查询返回不期望的结果

Question

我使用 rdflib python 库对联系人图进行建模，并执行 sparql 查询来检索谁认识谁。当人们添加为 URIRef 时，这可以正常工作，但在使用 BNode 时则不行。

示例图可以表示如下:

bob   - knows -> linda
alice - knows -> linda
tom   - knows -> linda
        knows -> bob

只有汤姆认识鲍勃，没有人认识汤姆。

我执行以下 2 个查询:

1. 第一个救出汤姆；它按预期工作。
2. 在第二个查询中，我使用 Tom 节点 id 来检索谁认识他。我期待一个空列表。当 Tom 添加为 URIRef 时，它会按预期工作。但是，当 Tom 添加为 BNode 时，此查询将返回 3 个名称!

use_blank_node = True # switch to see the undesired behavior happens only with blank node

pred_knows = URIRef("http://example.org/knows")
pred_named = URIRef("http://example.org/named")

def create_graph() -> Graph:
    graph = Graph()

    bob = URIRef("http://example.org/people/Bob")
    linda = BNode()  # a GUID is generated
    alice = BNode()
    tom = BNode() if use_blank_node else URIRef("http://example.org/people/Tom")
    print(f"{str(tom)=}")
    remy = BNode()

    graph.add((bob, pred_named, Literal("Bob")))
    graph.add((alice, pred_named, Literal("Alice")))
    graph.add((tom, pred_named, Literal("Tom")))
    graph.add((linda, pred_named, Literal("Linda")))
    graph.add((remy, pred_named, Literal("Remy")))

    graph.add((bob, pred_knows, linda))
    graph.add((alice, pred_knows, linda))
    graph.add((tom, pred_knows, linda))
    graph.add((tom, pred_knows, bob))

    return graph


find_tom_who_knows_bob_query = f"""SELECT DISTINCT ?knowsbob ?nameofwhoknowsbob
WHERE 
{{ ?knowsbob <{pred_knows}> <http://example.org/people/Bob> ;
             <{pred_named}> ?nameofwhoknowsbob . 
 }}"""


def find_who_know_tom(tom_id) -> str:
    tom_query = f"_:{tom_id}" if type(tom_id) is BNode else f"<{tom_id}>"

    return f"""SELECT DISTINCT ?nameOfWhoKnowsTom
    WHERE 
    {{ ?iriOfWhoKnowsTom  <{pred_knows}> {tom_query} ;
                          <{pred_named}> ?nameOfWhoKnowsTom}}"""


def main():
    graph = create_graph()
    print("=" * 60, "\n", graph.serialize(), "\n", "=" * 60)

    result = list(graph.query(find_tom_who_knows_bob_query))
    assert len(result) == 1 and len(result[0]) == 2
    tom_id = result[0][0]
    print(f"{str(tom_id)=}")
    assert (type(tom_id) == BNode and use_blank_node) or (type(tom_id) == URIRef and use_blank_node is False)
    assert str(result[0][1]) == "Tom"

    query = find_who_know_tom(tom_id)
    print(query)
    result = list(graph.query(query))
    print(
        "They know Tom:", ", ".join([str(r[0]) for r in result])
    )  # why is it not empty when use_blank_node = True
    # prints: "They know Tom: Bob, Alice, Tom"


if __name__ == "__main__":
    main()

我的问题:如何正确使用 sparql 以便查询也适用于空白节点？

Answer 1

空白节点类似于自由变量。来自 SPARQL 1.1 Query Language :

An application writer should not expect blank node labels in a query to refer to a particular blank node in the data.

您的第二个查询，而不是

SELECT ?nameOfWhoKnowsTom WHERE {
   ?iriOfWhoKnowsTom ex:knows _:N6fb3b031995c43cfbf3e257ec0c0eac0 ;
                     ex:named ?nameOfWhoKnowsTom .
}

应该是这样的:

SELECT ?nameOfWhoKnowsTom WHERE {
   ?iriOfWhoKnowsTom ex:knows / ex:named "Tom" ;
                     ex:named ?nameOfWhoKnowsTom .
}

另请参阅blank-nodes标签信息。