Write an aggregation with $match and $unwind

Working with Data
,
#1

Hi everyone! I need help with writing an aggregation query.

Items collection:

{ 
_id,
supplierId: { type: Schema.Types.ObjectId, ref: 'Supplier', required: true}
UOMs: [
      {
        uomCode: { type: Schema.Types.ObjectId, ref: 'UOM', required: true },
        packSize: { type: Number, required: true },
        //other fields
      }
// other fields
],
}

UOM collection:

{
_id: UOM_id,
isActual: { type: Boolean, required: true },
width:  { type: Number }
//other fields
}

What I intend to do:
I receive a query with supplierId in a header and a queryCode as a query parameter, and filter it using $match by supplierId. Then I want to populate uomCode field in every object in UOMs array from UOM collection, $unwind the items based on UOMs array and filter the result: select only unwound items with isActual: true and UOM_id === queryCode. I hope this is possible.

My current query looks as this:

const aggregateArray = [
  {
    $match: itemQuery
  },
  {
    $unwind: {
      path: '$UOMs'
    }
  },
  {
    $lookup: {
      from: 'inv_uoms',
      localField: 'UOMs.uomCode',
      foreignField: '_id',
      let: { id: '$UOMs._id' },
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [ $eq: { ['$id', queryCode] }, $eq: { ['$isActual', true] } ]
            }
          }
        }
      ],
      as: 'UOMs.uomCode'
    }
  },
  {
    $project: {
      supplierId: 1,
      UOMs: 1
      //other fields
    }
  }
];

const items = await Item.aggregate(aggregateArray);

Currently I receive an array of items with UOMs as an array containing one populated object but uomCode is an EMPTY array. What am I doing wrong?

Thanks for your help!

#2

One error that is often made is that the query value is exchanged as string so it does not match the stored value. The latter being an ObjectId. You might simply need to create an ObjectId from queryCode.