Mongodb and Mongoose putting many id for relationship

Question

So I have Client and Vehicle model, so what I want to do is add many Vehicles to the Client (many ids) but I don't know how to do it, it works fine with one but not with two. I tried some anwsers with array but seems is not working.

Here is the route:

router.post('/create', passVerify, async (req, res) => {
  try {
    var vehicles = null;
    var orders = null;
    const body = req.body;
    if (req.body.vehicles) vehicles = toId(req.body.vehicles);
    if (req.body.vehicles) orders = toId(req.body.orders);
    const save = new Client({ ...body, vehicles, orders });
    const client = await save.save();
    if (client)
      res.status(200).send({
        success: true,
        data: client,
      });
    else
      res.status(404).send({
        success: false,
        message: 'Something went wrong while adding the client to database.',
      });
  } catch (error) {
    res.status(500).send({
      success: false,
      message: error,
    });
    console.log(error);
  }
});

And here is the part of the model:

vehicles: [
      {
        type: Types.ObjectId,
        ref: 'vehicle',
      },
    ],