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

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