public function testReadSecondaryOnly(array $tags = null)
{
$this->collection->readSecondaryOnly(array(array('dc' => 'kyiv'), array('dc' => 'lviv')));
$this->assertEquals(array('type' => \MongoClient::RP_SECONDARY, 'tagsets' => array(array('dc' => 'kyiv'), array('dc' => 'lviv'))), $this->collection->getReadPreference());
}