How to unset fields based on the fields' name with a "less than" condition?

Question

I am trying to unset certain fields based on the name of the field (the key).

Say I have something like this:

{
    5: "cool",
    93: "cool",
    30: "cool",
    56: "cool"
}

How would I unset all fields with a value below, say, 40.

So the result should be:

{
    5: "cool",
    30: "cool"
}

I tried using the less than command on the whole field or using the positional operator but both of those failed.

collection.update_one(
        {"_id": id},
        {"$unset": {"blacklist.$": {"$lt": 40}}}
    )

I couldn't find anything online or on the docs so I am hoping to find some help here,

Thanks!

Answer 1

You can't really use $unset like this, we can still achieve this using pipelined updates - with slightly more complicated syntax.

Our approach will be to turn our root object to an array using $objectToArray, iterate over it and filter all numeric keys under a certain threshold. then finally convert back to an object and update our document, like so:

db.collection.update({},
[
  {
    $replaceRoot: {
      newRoot: {
        $arrayToObject: {
          $filter: {
            input: {
              $objectToArray: "$$ROOT"
            },
            cond: {
              $cond: [
                {
                  $regexMatch: {
                    input: "$$this.k",
                    regex: "^[0-9]+$"
                  }
                },
                {
                  $lt: [
                    {
                      $toInt: "$$this.k"
                    },
                    40
                  ]
                },
                true
              ]
            }
          }
        }
      }
    }
  }
])

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