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<div class="titlepage"><div><div><h2 class="title">
<a name="tracker-examples-writeonly-with-blank-nodes"></a>Updating the Store with Blank Nodes</h2></div></div></div>
<p>
      The majority of the work here is already described in the
      previous example where we talk about how to write the store.
    </p>
<p>
      The difference with this example is that sometimes you want to
      insert data and have the URNs returned which were created to
      avoid re-querying for them. This is done using
      the <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-update-blank" title="tracker_sparql_connection_update_blank ()">tracker_sparql_connection_update_blank</a></code> function (or asynchronously <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-update-blank-async" title="tracker_sparql_connection_update_blank_async ()">tracker_sparql_connection_update_blank_async</a></code>).
      If launched asynchronously, the result of the operation can be obtained with
      <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-update-blank-finish" title="tracker_sparql_connection_update_blank_finish ()">tracker_sparql_connection_update_blank_finish</a></code>.
    </p>
<p>
      The <span class="emphasis"><em>_:foo</em></span> in the example is how a blank node is
      represented in SPARQL. The <span class="emphasis"><em>foo</em></span> part is used to generate the
      unique ID that is used for the new URN. It is also used in the
      <code class="function"><a href="/usr/share/gtk-doc/html/glib/glib-GVariant.html#GVariant">GVariant</a></code>
      that is returned. In the example below, we are creating a new
      blank node called <span class="emphasis"><em>foo</em></span> for every class that
      exists.
    </p>
<p>
      The format of the GVariant (in D-Bus terms) is an aaa{ss} (an
      array of an array of dictionaries). This is rather complex but
      for a good reason. The first array represents each INSERT that
      may exist in the SPARQL. The second array represents each new
      node for a given WHERE clause (the example below illustrates
      this), you need this to differentiate between two INSERT
      statments like the one below in the same SPARQL sent to the
      store. Last, we have a final array to represent each new node's
      name (in this case <span class="emphasis"><em>foo</em></span>) and the actual URN which was
      created. For most updates the first two outer arrays will only
      have one item in them.
    </p>
<p>
      The following program shows how a synchronous blank node update can be done to the store:

</p>
<pre class="programlisting">
#include &lt;tracker-sparql.h&gt;

int main (int argc, const char **argv)
{
  GError *error = NULL;
  GVariant *v;
  <span class="type"><a class="link" href="TrackerSparqlConnection.html#TrackerSparqlConnection-struct" title="struct TrackerSparqlConnection">TrackerSparqlConnection</a></span> *connection;
  const gchar *query =
    "INSERT { _:foo a nie:InformationElement } WHERE { ?x a rdfs:Class }";

  /* Do NOT get a direct connection if you're going to do some write
   * operation in the Store. The NULL represents a possible
   * GCancellable.
   */
  connection = <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-get" title="tracker_sparql_connection_get ()">tracker_sparql_connection_get</a></code> (NULL, &amp;error);
  if (!connection) {
    g_printerr ("Couldn't obtain a connection to the Tracker store: %s",
                error ? error-&gt;message : "unknown error");
    g_clear_error (&amp;error);

    return 1;
  }

  /* Run a synchronous blank node update query */
  v = <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-update-blank" title="tracker_sparql_connection_update_blank ()">tracker_sparql_connection_update_blank</a></code> (connection,
  				              query,
  				              G_PRIORITY_DEFAULT,
  				              NULL,
  				              &amp;error);

  if (error) {
    /* Some error happened performing the query, not good */
    g_printerr ("Couldn't update the Tracker store: %s",
                error ? error-&gt;message : "unknown error");

    g_clear_error (&amp;error);
    g_object_unref (connection);

    return 1;
  }

  if (!v) {
    g_print ("No results were returned\n");
  } else {
    GVariantIter iter1, *iter2, *iter3;
    const gchar *node;
    const gchar *urn;

    g_print ("Results:\n");

    g_variant_iter_init (&amp;iter1, v);
    while (g_variant_iter_loop (&amp;iter1, "aa{ss}", &amp;iter2)) { /* aa{ss} */
      while (g_variant_iter_loop (iter2, "a{ss}", &amp;iter3)) { /* a{ss} */
        while (g_variant_iter_loop (iter3, "{ss}", &amp;node, &amp;urn)) { /* {ss} */
	  g_print ("  Node:'%s', URN:'%s'\n", node, urn);
	}
      }
    }

    g_variant_unref (v);
  }

  g_object_unref (connection);

  return 0;
}
</pre>
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