Why is my mongoose populate not retrieving the nested array? - mongodb

Question

I have the following mongoose schemas:
const CategoriesSchema = new mongoose.Schema(
{
sites: [
{
site: {
type: ObjectId,
ref: "Sites",
}
}
],
);
mongoose.model("Categories", CategoriesSchema, 'organizer-categories');
And:
const SitesSchema = new Schema({
url: {
type: String,
required: true,
},
img: {
type: String,
},
});
mongoose.model("Sites", SitesSchema, "organizer-sites");,
The following function is supposed to create a new site in mongodb, add the site to Categories as shown in the schema and return the updated category with the new site data using mongoose's populate method. But only the _id of site is being returned with the category data.
exports.addSite = async (req, res) => {
const {categoryId, siteUrl} = req.body;
console.log('categoryId, siteUrl', categoryId, siteUrl)
try {
let site = await Sites.findOne({url: siteUrl});
console.log('site', site)
if(site) {
return res.status(422).send(`${siteUrl} already exists.`)
}
site = await Sites.create({url: siteUrl, categoryId});
// site = {url: siteUrl, categoryId};
const category = await Categories.findOneAndUpdate({
_id: categoryId
}, {
$addToSet: {
sites: site
}
}, {
new: true
}).populate({
path: 'sites.site',
model: 'Sites'
})
console.log('category', category)
res.status(201).json(category);
} catch(error) {
console.error(error);
return res.status(500).send('Error adding category.')
}
}