public function testGroupWithOperatorInId()
{
$groupStage = new Group($this->getTestAggregationBuilder());
$groupStage->field('_id')->year('$dateField')->field('count')->sum(1);
$this->assertSame(['$group' => ['_id' => ['$year' => '$dateField'], 'count' => ['$sum' => 1]]], $groupStage->getExpression());
}