Sokil\Mongo\Pipeline\GroupStageTest::testSum_ArrayExpression PHP Method

    public function testSum_ArrayExpression()
    {
        $pipeline = new Pipeline($this->collection);
        $pipeline->group(function ($stage) {
            /* @var $pipeline \Sokil\Mongo\Pipeline\GroupStage */
            $stage->setId('$userId')->sum('totalAmount', array('$add' => array('$amount', 12)));
        });
        $this->assertEquals('[{"$group":{"_id":"$userId","totalAmount":{"$sum":{"$add":["$amount",12]}}}}]', (string) $pipeline);
    }