mongodb project map fields to return from query with wildcard

I’m working on a system where I use a schema with the type Map much like this:

const Product = mongoose.model('Product', {
    name: String,
    data:{
        type:Map,
        of:String
    }
});

I’m trying to query certain parts of the docs in this collection with a projection:

Product.findOne({
    _id: req.params.id
}, {
   name:true,
   data.xy*
})

I would like to only get the fields in data starting with xy. Do you have any hint how to do that?

Answer

Since Mongoose’s map type will be represented as a nested object in mongodb, you could convert using $objectToArray which will result in array of key-value pairs that allows you to query by the keys starting with xy. Something like:

Product.aggregate([
  {
    $project: {
      data: {
        $objectToArray: "$data"
      }
    }
  },
  {
    "$unwind": "$data"
  },
  {
    $match: {
      "data.k": {
        $regex: "^xy"
      }
    }
  }
])

Here’s a working example on mongoplayground:

https://mongoplayground.net/p/yCJLhzalOXI