Add or update field with random numbers ($merge, $rand, $floor, $set)

db.getCollection('collection-name').aggregate([
    {
        $match: { _id: ObjectId("60c08f97f4738c0018701eb9") }
        // other conditions ...
    },
    {
        $set: {
            // MongoDB will creates this field if there is not
            fieldName: {
                // this operators will generate random numbers from 0 to 20 for this field
                $floor: {
                    $multiply: [ { $rand: {} }, 20 ]
                }
            }
        }
    },
    {
        $merge: {
            into: 'contractors',
            on: '_id',
            whenMatched: 'merge',
        }
    }
])