Example 4: Statement matching¶
The getStatements() method of the connection object provides a
simple way to perform unsophisticated queries. This method lets you
enter a mix of required values and wildcards, and retrieve all
matching triples. (If you need to perform sophisticated tests and
comparisons you should use a SPARQL query instead.)
Below, we illustrate two kinds of getStatements() calls. The
first mimics traditional RDF4J syntax, and returns a Statement
object at each iteration. We will reuse functions defined in previous
examples to create a connection object and populate the repository
with four triples describing Bob and Alice. We’re going to search for
triples that mention Alice, so we have to create an “Alice” URI to use
in the search pattern:
conn = connect()
add_bob_and_alice(conn)
alice = conn.createURI("http://example.org/people/alice")
Now we search for triples with Alice’s URI in the subject
position. The None values are wildcards for the predicate and
object positions of the triple.
statements = conn.getStatements(alice, None, None)
The getStatements() method returns a RepositoryResult object
(bound to the variable statements in this case). This object can
be iterated over, exposing one result statement at a time. It is
sometimes desirable to screen the results for duplicates, using the
enableDuplicateFilter() method. Note, however, that duplicate
filtering can be expensive. Our example does not contain any
duplicates, but it is possible for them to occur.
with statements:
statements.enableDuplicateFilter()
for statement in statements:
print(statement)
This prints out the two matching triples for “Alice.”
(<http://example.org/people/alice>, <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>, <http://example.org/ontology/Person>)
(<http://example.org/people/alice>, <http://example.org/ontology/name>, "Alice")
Notice how we used the with keyword to ensure that the
RepositoryResult object is closed after the results are
fetched. This is necessary to release resources used during result
retrieval. The same goal could be accomplished by calling the
RepositoryResult.close() method (preferably in a finally
block to ensure exception safety).
