[{"_id":"grandParentId1","types":[{"_id":"parentId1","files":[{"url":"1.example.com","_id":"1childId"},{"url":"2.example.com","_id":"2childId"}]},{"_id":"parentId11","files":[{"url":"1.example.com","_id":"1childId"},{"url":"20.example.com","_id":"20childId"}]}]},{"_id":"grandParentId2","types":[{"_id":"parentId","files":[{"url":"1.example.com","_id":"1childId"},{"url":"2.example.com","_id":"2childId"}]}]}]

db.collection.aggregate([{$match:{"types.files._id":"2childId"}},{$unwind:"$types"},{$unwind:"$types.files"},{$replaceRoot:{newRoot:"$types.files"}},{$match:{_id:"2childId"}}])