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

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