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