Requiring a query to determine the length of a document's array and calculate the appropriate positive integer index introduces a race condition. By allowing negative indexes (indicating distance from the end) on the $position modifier, this race condition could be avoided.

This would match Python semantics for list manipulation. An example, given the following document:

{
  acl: [
      {grant: True, user: "amcgregor"},
      {grant: True, user: "bdole"},
      {grant: False}
    ]
}

And the following update operation:

{
  $push: {
    acl: {
       $each: [ { grant: True, user: "algore" } ],
       $position: -1
    }
  }
}

The resulting document should be:

{
  acl: [
      {grant: True, user: "amcgregor"},
      {grant: True, user: "bdole"},
      {grant: True, user: "algore"},
      {grant: False}
    ]
}

The default insert location, the end (when $position is omitted or exceeds the length of the array), is unaffected.

Thank you for your consideration!