Database
[{"_id":"1","elem":"ok","arrayOfObjects":[{"type":"Object1","fieldx":"lol"},{"type":"SndObject","fieldy":"foo"},{"type":"Object1","fieldx":"bob"}]}]
Query
db.collection.aggregate([{$unwind:"$arrayOfObjects"},{$group:{_id:{type:"$arrayOfObjects.type",_id:"$_id"},elem:{$first:"$elem"},arrayOfObjects:{$push:"$arrayOfObjects"}}},{$group:{_id:"$_id._id",elem:{$first:"$elem"},arrayOfObjects:{$mergeObjects:{$arrayToObject:[[{k:"$_id.type",v:"$arrayOfObjects"}]]}}}}])