public function testProjectStage()
{
$projectStage = new Project($this->getTestAggregationBuilder());
$projectStage->excludeIdField()->includeFields(array('$field', '$otherField'))->field('product')->multiply('$field', 5);
$this->assertSame(array('$project' => array('_id' => false, '$field' => true, '$otherField' => true, 'product' => array('$multiply' => array('$field', 5)))), $projectStage->getExpression());
}