Sokil\Mongo\Pipeline\ExpressionTest::testAdd_CallableExpressin PHP Method

    public function testAdd_CallableExpressin()
    {
        $pipeline = new Pipeline($this->collection);
        $pipeline->group(function ($stage) {
            /* @var $stage \Sokil\Mongo\Pipeline\GroupStage */
            $stage->setId('user.id')->sum('totalAmount', function ($expression) {
                /* @var $expression \Sokil\Mongo\Pipeline\Expression */
                $expression->add(array(function ($expression) {
                    $expression->multiply('$amount', 0.95);
                }, 3.15));
            });
        });
        $this->assertEquals('[{"$group":{"_id":"user.id","totalAmount":{"$sum":{"$add":[{"$multiply":["$amount",0.95]},3.15]}}}}]', (string) $pipeline);
    }