Query MongoDb aggregate unisce due raccolte

devi solo dichiarare $someField nella sezione let.

db.collectionA.aggregate([
  {
    $lookup: {
      from: 'collectionB',
      let: { some_field: '$someField' },
      pipeline: [
        { $match: {
            $expr: {
              $and: [
                { $eq: [ "$someField", "$$some_field" ] },
                { $eq: [ "$otherField", "789" ] }
              ]
            }
          }
        }
      ],
      as: 'B'
    }
  },
  {
    $match: {
      $expr: {
        $gt: [ { $size: "$B" }, 0 ]
      }
    }
  }
])

https://mongoplayground.net/p/RTiUMWl8QaX