How do I slice a nested array and how do I update a specific field in it?

Working with Data
9

So for the first part, here’s my sample data (I changed count to comments, because that’s the real field):

{
    "_id" : "ROOT_DOCUMENT_ID",
    "array1" : [ 
        {
            "_id" : "ARRAY1_A",
            "array2" : [ 
                {
                    "_id" : "ARRAY2_A",
                    "comments" : 0
                }
            ]
        }, 
        {
            "_id" : "ARRAY1_B",
            "array2" : [ 
                {
                    "_id" : "ARRAY2_A",
                    "comments" : 10
                }, 
                {
                    "_id" : "ARRAY2_B",
                    "comments" : 20
                }, 
                {
                    "_id" : "ARRAY2_C",
                    "comments" : 30
                }
            ]
        }
    ]
}

When I run the following find:

db.aggregate([
    { $match: { "$expr": { "$eq": [ "$_id", "ROOT_DOCUMENT_ID" ] } } },
    {
        $project: {
            "_id": 0,
            "comments": {
                $let: {
                    "vars": {"array1": "$$ROOT.array1" },
                    "in": {
                    $let: {
                        "vars": {
                        "array1_element": {
                            $arrayElemAt: [
                            {
                                $filter: {
                                "input": "$$array1",
                                "as": "m",
                                "cond": {
                                    $eq: [
                                    "$$m._id",
                                    "ARRAY1_B"
                                    ]
                                }
                                }
                            },
                            0
                            ]
                        }
                        },
                        "in": {
                        $let: {
                            "vars": {
                            "array2": "$$array1.array2"
                            },
                            "in": {
                            $let: {
                                "vars": {
                                "array2": {
                                    $arrayElemAt: [
                                    "$$array2",
                                    0 // This is the index I want
                                    ]
                                }
                                },
                                "in": "$$array2.comments"
                            }
                            }
                        }
                        }
                    }
                    }
                }
            }
        }
    }
])

It returns:

{
  comments: [10, 20, 30]
}

It returns the same thing even when I change the index to 1 or 2. What I’m expecting is:

{
  comments: 10
}

And for the 3 methods - is method #3 the best, since method 1 doesn’t allow me to update at the first and last index?