Mongoose find by ObjectID reference

I have two schemas:

    let adSchema = mongoose.Schema (
    {
        author: {type: ObjectId, ref: 'User'},
        title: {type: String, required: true },
        town: {type: ObjectId, ref: 'Town', required: true},
    }
);

And town schema:

    let townSchema = mongoose.Schema (
    {
        name: {type: String, required:true, unique:true},
        ads: [{type: ObjectId, ref: 'Ad'}]
    }
);

I need to make query that finds all Ads by town.name How can I do that?

via petko kostadinov