Mongoose populate returns empty array or list of ObjectIds

I am practicing my express.js skills by building a relational API and am struggling to populate keys in a schema.

I am building it so I have a list of properties, and those properties have units. The units have a propertyId key.

This is currently returning an empty array, whereas if i remove the populate({}) it returns an array of ObjectIds.

I've read a number of posts and some people solved this by using .populate({path: 'path', model: Model}); but this doesn't seem to be doing the trick. I think it might be the way I am adding a propertyId to the unit but I'm not sure. Can anyone see where I am going wrong? Any help will be massively appreciated.

Here are the schemas.

Property:

const mongoose = require('mongoose');
const { Schema } = mongoose;

const PropertySchema = new Schema({
  title: {
    type: String,
    required: true
  },
  date: {
    type: Date,
    default: Date.now
  },
  units: [
    {
      type: mongoose.Schema.Types.ObjectId,
      ref: 'unit'
    }
  ]
});

module.exports = Property = mongoose.model('property', PropertySchema);

Unit:

const mongoose = require('mongoose');
const { Schema } = mongoose;

const UnitSchema = new Schema({
  title: {
    type: String,
    required: true
  },
  propertyId: {
    type: Schema.Types.ObjectId,
    ref: 'property'
  }
});

module.exports = Unit = mongoose.model('unit', UnitSchema);

I am then creating the unit like this:

-- api/properties/:id/units --
router.post('/:id/units', async (req, res) => {

  // Get fields from req.body
  const { title } = req.body;

  // Get current property
  const property = await Property.findById(req.params.id);

  try {

    // Throw error if no property
    if (!property) {
      return res.status(400).json({ msg: 'Property not found' });
    }

    // Create new unit
    const newUnit = new Unit({
      title,
      propertyId: req.params.id
    });

    // Add new unit to property's units array
    property.units.unshift(newUnit);

    // Save property
    await property.save();

    // Return successful response
    return res.status(200).json(property);

  } catch (error) {
    console.error(error.message);
    return res.status(500).send('Server error');
  }
});

And trying to populate in the GET request

-- /api/properties/:id/units --
const Unit = require('../../models/Unit');

router.get('/:id/units', async (req, res) => {
  const property = await Property.findOne({ _id: req.params.id }).populate({path: 'units', model: Unit});
  const propertyUnits = property.units;
  return res.status(200).json(propertyUnits);
});

If i remove the .populate({path: 'units', model: Unit});, I get a list of unit id's like this:

[
    "5ff7256cda2f5bfc1d2b9108",
    "5ff72507acf9b6fb89f0fa4e",
    "5ff724e41393c7fb5a667dc8",
    "5ff721f35c73daf6d0cb5eff",
    "5ff721eb5c73daf6d0cb5efe",
    "5ff7215332d302f5ffa67413"
]