how to use let in $lookup aggregation Mongodb

Sergio Cano :

i am trying to query some data using aggregation but something is not working...

I've been searching some examples and this is what i have...

( From my web app i just recieve a searchTerm )

 productosModel.aggregate([
        //IN THE FIRST STAGE I FILTER BY REF OR NAME ( searchTerm )
        {
            $match: {
                $or: [
                    { ref: { '$regex': req.body.ref, '$options': 'i' } },
                    { nombre: { '$regex': req.body.nombre, '$options': 'i' } }
                ]
            }
        },
        {
            $lookup: {
                from: "configtarifas",
                let: { "producto": "$_id" }, //HERE IS WHERE I PRETEND TO DEFINE THE _ID FILTERED 
                pipeline: [
                    {
                        $match: {
                            $expr: { $eq: ['$idProducto',  "$$producto" ] },
                        }
                    }
                ],
                as: "configs",
            }
        }
    ])

Then i need to use $lookup to join this collection and add configTarifas to the data, but i would like receive just the documents that match with the _id of the product filtered before...

what am i doing wrong? the config collection is not matching any $$producto... seemns like $$producto is not getting the _id value

Valijon :

You have a mismatched types between parent $_id (ObjectId) field and idProducto (String). You need to convert ObjectId to String with $toString or $convert operators

{
  $lookup: {
    from: "configtarifas",
    let: { "producto": {$toString: "$_id"} },
    pipeline: [
      {
        $match: {
          $expr: { $eq: ['$idProducto',  "$$producto" ] },
        }
      }
    ],
    as: "configs",
  }
}