Sokil\Mongo\Database::readSecondaryOnly PHP Method

    public function readSecondaryOnly(array $tags = null)
    {
        $this->getMongoDB()->setReadPreference(\MongoClient::RP_SECONDARY, $tags);
        return $this;
    }

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