public function checkOut(User $user, Library $library, $circ_desk = 'DEFAULT_CIRC_DESK')
    {
        $postData = [
            'library'   => ['value' => $library->code],
            'circ_desk' => ['value' => $circ_desk],
        ];

        $data = $this->client->postJSON(
            $this->url('/loans', ['user_id' => $user->id]),
            $postData
        );

        return Loan::make($this->client, $user, $data->loan_id)
            ->init($data);
    }