public function havePositions(array $req_positions)
    {
        $positions = new ArrayCollection();

        foreach ($req_positions as $position) {
            if ($position = trim($position) === '') {
                continue;
            }
            if ($positions->contains($position)) {
                continue;
            }

            $positions->add($position);
        }

        if (!$positions->isEmpty()) {
            $this->positions = $positions;
        }

        return $this;
    }