Hello,
We have schema in one of our collection called 'assets' as following

{
"_id" : "002992f0b34241a38f6c2336b52c480b",
"storage" : {
"size" : 0,
"sha256" : null,
"account_id" : "bab7a257b8db102d513eee5c279522aa",
"updated" : NumberLong("1439473126075068"),
"revision" :

{ "_id" : "da3b4547944f48fda7ea542cbc5d0e43", "source" : "importSource.fileSize" }

},
"created" : NumberLong("1439473126075068"),
"subtype" : "video"
"updated" : NumberLong("1439473126075068"),
"catalog_id" : "35575220144c48e98f158cac91b5e5f9",
"payload" :

{ "captureDate" : "2015-08-13T19:08:45Z", ..... }

,
....
}

We want to group the documents in this collection by subtype and planning to use query something like:

db.assets.aggregate({ $match: { catalog_id: "35575220144c48e98f158cac91b5e5f9", deleted: {$exists: false}}} , { $group:

{ _id: "$subtype"}

});

However, the "subtype" field may not be present in all the documents and we want to group such documents under one category (images). How can we achieve this using aggregation framework ?