{
    "_id" : ObjectId(someId),
    sourceId: someId,
    "objArr" : [
        {
            "fieldA" : someValue,
            "fieldB" : someValue
        },
        {
            "fieldA" : someValue,
            "fieldB" : someValue
        },
        ...etc
    ]
}

db.table.aggregate([
  {$match:{_id:1}},
  {$unwind:"$coll"},
  {$match:{"coll.fieldA":{$gt:10},"coll.fieldB":{$lt:20}}},
  {$project:{"fieldA":"$coll.fieldA","fieldB":"$coll.fieldB"}}
])