It would be nice if the $size operator supported objects!

This would specifically be handy for implementing the update-if-current idom; e.g. one could match the following subdocument exactly and order-indipentantly:

Document:

{
  _id: 1,
  my_doc: {
    name: "Ford Prefect",
    occupation: "Writer"
  }
}

Query:

{
  _id: 1,
  "my_doc.name": "Ford Prefect",
  "my_doc.occupation": "Writer",
  my_doc: {$size: 2}
}