Filtering array items from items in another array in mongodb aggregation

Question

I have managed to create an aggregate which returns two array in a document like so:

"b": [
                {
                    "_id": "6258bdfe983a2d31e1cc6a4b",
                    "booking_room_id": "619395ba18984a0016caae6e",
                    "checkIn_date_time": "2022-04-16",
                    "checkOut_date_time": "2022-05-17"
                }
            ]

"r": [
                {
                    "_id": "619395ba18984a0016caae6e",
                }
            ]

I want to remove the item from r if _id in r matches booking_room_id in b.

Also, since these array exist inside a parent document. I want to remove the parent document from the query if r is empty after performing the filter.

Answer 1

Use $expr and $filter

db.collection.aggregate([
  {
    $match: {
      $expr: {
        $ne: [
          {
            $filter: {
              input: "$r",
              as: "r",
              cond: {
                $not: { $in: [ "$$r._id", "$b.booking_room_id" ] }
              }
            }
          },
          []
        ]
      }
    }
  },
  {
    $set: {
      r: {
        $filter: {
          input: "$r",
          as: "r",
          cond: {
            $not: { $in: [ "$$r._id", "$b.booking_room_id" ] }
          }
        }
      }
    }
  }
])

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