zendframework 2 select max of a column - select

Question

I'm using ZendFramework 2 and TableGateway which is fine for normal select statements. But I can't seem to find how to get a max of a column using ZendFramework 2 select.
My select should look something like
SELECT MAX( `publication_nr` ) AS maxPubNr FROM `publications`
and my code looks like:
use Zend\Db\TableGateway\TableGateway;
class PublicationsTable
{
protected $tableGateway;
...
public function getMaxPubicationNr()
{
$rowset = $this->tableGateway->select(array('maxPubNr' => new Expression('MAX(publication_nr)')));
$row = $rowset->current();
if (!$row) {
throw new \Exception("Could not retrieve max Publication nr");
}
return $row;
}

Answer 1

If you look at the TableGateway you will notice the parameters for the Select method are actually passed to the Where part of the Query, which is making your query incorrect.
You need to modify the Select Object directly as the TableGateway won't give you any proxy methods to do this.
You could try something like this:
public function getMaxPubicationNr()
{
$select = $this->tableGateway->getSql()->select();
$select->columns(array(
'maxPubNr' => new Expression('MAX(publication_nr)')
));
// If you need to add any where caluses you would need to do it here
//$select->where(array());
$rowset = $this->tableGateway->selectWith($select);
$row = $rowset->current();
if (!$row) {
throw new \Exception("Could not retrieve max Publication nr");
}
return $row;
}
I've not tested it, but that should just about get you there :)
You will probably bump into another issue though, and that will be due to the TableGateway trying to build you an object from the result, but you aren't bringing back a full row to build an object, you're just bringing back a single Column.
I would just add use the Db/Select objects to do this and not bother with the GateWay to be honest, I don't think it's supposed to be used like this.