If I'm grouping by something, unless I explicitly specify an accumulator for every field, that field will be lost. The only work around that exists right now is to re-query for the document by its _id and fill in the fields.

There should be some way to specify a $default accumulator that's performed on any unspecified field. For example:

{ $group: { _id: '$_id', number:

, $default: {"defaulted$property": {$first:$property}}} }

The above would group by _id, accumulate the number property as a sum, and for every other property it would simply add in the first value in that group for that property as the name "defaulted"+propertyName