public function create_schema_version_table()
{
if (!$this->has_table($this->get_schema_version_table_name())) {
$t = $this->create_table($this->get_schema_version_table_name(), array('id' => false));
$t->column('version', 'string');
$t->finish();
$this->add_index($this->get_schema_version_table_name(), 'version', array('unique' => true));
}
}