How to get the maximum of sections.Id in below document where collection.\_id = some parameter

```
{
    "_id" : ObjectId("571c5c87faf473f40fd0317c"),
    "name" : "test 1",
    "sections" : [ 
        {
             "Id" : 1,
             "name" : "first section"
        }, 
        {
            "Id" : 2,
            "name" : "section 2"
        }, 
        {
            "Id" : 3,
            "name" : "section 3"
        }
}

```

I have tried below 

```
db.collection.aggregate(
[
    {
        "$match": {
            "_id": ObjectId("571c5c87faf473f40fd0317c")
        }
    },
    {
        "$group" : {
            "_id" : "$_id",
            "maxSectionId" : {"$max" : "$sections.Id"}
        }
    }
]);

```

But instead of returning max int single value it is returning an array of all Ids in sections array. 

Further same query when executed in node.js it returns an empty array.