Doctrine decode function - oracle

Question

In my Symfony2 project, I want to get the sum of a column (Field1) only for some lines (Field2 = 1):
id Field1 Field2
1 10 1
2 20 1
3 30 0
4 40 1
5 50 0
In this example, I want a result of 70 (sum of lines 1, 2 and 4).
Since I Work with Oracle, I have this query working well in my SQL GUI :
SELECT
SUM(decode(Field2, 1, Field1, 0)) AS total
FROM myTable
Since Doctrine doesn't recognize Oracle decode function, I Tried to create mine :
namespace MyBundle\DQL;
use Doctrine\ORM\Query\Lexer;
use Doctrine\ORM\Query\SqlWalker;
use Doctrine\ORM\Query\Parser;
use Doctrine\ORM\Query\AST\Functions\FunctionNode;
/**
* "DECODE" "(" Expression "," Search "," Result "," Default ")"
*/
class DecodeFunction extends FunctionNode
{
public $expression;
public $search;
public $result;
public $default;
public function getSql(SqlWalker $sqlWalker)
{
return 'decode(' . $this->expression->dispatch($sqlWalker) . ','
. $this->search->dispatch($sqlWalker) . ','
. $this->result->dispatch($sqlWalker) . ','
. $this->default->dispatch($sqlWalker) . ')';
}
public function parse(Parser $parser)
{
$parser->match(Lexer::T_IDENTIFIER);
$parser->match(Lexer::T_OPEN_PARENTHESIS);
$this->expression = $parser->StringPrimary();
$parser->match(Lexer::T_COMMA);
$this->search = $parser->StringPrimary();
$parser->match(Lexer::T_COMMA);
$this->result = $parser->StringPrimary();
$parser->match(Lexer::T_COMMA);
$this->default = $parser->StringPrimary();
$parser->match(Lexer::T_CLOSE_PARENTHESIS);
}
}
config.yml
doctrine:
orm:
entity_managers:
default:
auto_mapping: true
...
dql:
string_functions:
decode: MyBundle\DQL\DecodeFunction
And my querybuilder :
$qb = $this->createQueryBuilder('e');
$qb
->select('SUM(decode(e.Field2, :expect, e.Field1, :default)')
->setParameter('expect', 1)
->setParameter('default', 0);
But I got this error :
[Semantical Error] line 0, col 62 near 'Field2,': Error: Invalid PathExpression. Must be a StateFieldPathExpression.
So what I'm doing wrong?
Maybe is there a simpler solution?
Thanks