Operators

$expr

Allows comparisons between fields of the same document.

{
  "$expr": {
    "<aggregationOperator>": [ "<field1>", "<field2>" ]
  }
}

$exists

Checks if a field exists

{
  "field":{"$exists" : true}
}

$rename

Rename a field

{
  "$rename" : {"old_name":"new_name"}
}

$set

Sets a field

{
  "$set" : {"field":"value","field2":"value"}
}

$unset

Removes a field

{
  "$unset" : ["field1","field2"]
}