With MongoDB 3.2 version and newer, the $unwind
operator now has some options where in particular the preserveNullAndEmptyArrays
option will solve this.
If this option is set to true and if the path is null, missing, or an empty array, $unwind
outputs the document. If false, $unwind
does not output a document if the path is null, missing, or an empty array. In your case, set it to true:
db.collection.aggregate([
{ "$unwind": {
"path": "$solved",
"preserveNullAndEmptyArrays": true
} },
{ "$group": {
"_id": "$_id",
"login": { "$first": "$login" },
"solved": { "$sum": "$solved.points" }
} }
])