Doctrine\MongoDB\Query\Builder::expr PHP Method

    public function expr()
    {
        return new Expr();
    }

 /**
  * @param Builder                $queryBuilder
  * @param ConditionNodeInterface $node
  * @param Expr                   $expr
  * @return null
  */
 protected function computeExpression(Builder $queryBuilder, ConditionNodeInterface $node, Expr $expr = null)
 {
     if (count($node->getFields()) == 0 && count($node->getChildren()) == 0) {
         return null;
     }
     $method = $node->getOperator() === ConditionNodeInterface::EXPR_AND ? 'addAnd' : 'addOr';
     $expression = null === $expr ? $queryBuilder : $expr;
     $count = 0;
     foreach ($node->getFields() as $condition) {
         if (null !== $condition) {
             /** @var ConditionInterface $condition */
             $expression->{$method}($condition->getExpression());
             $count++;
         }
     }
     foreach ($node->getChildren() as $child) {
         $subExpr = $queryBuilder->expr();
         $subCount = $this->computeExpression($queryBuilder, $child, $subExpr);
         if ($subCount > 0) {
             $expression->{$method}($subExpr);
             $count++;
         }
     }
     return $count;
 }