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

Working with Data
11

For the first one - good catch! That was actually just a typo in my post here. But I found out what the real problem was. The last condition.

"in": "$$array2.comments"

Should be:

"in": "$$array2_member.comments"

Final working query is here:

db.aggregate([
    { $match: { "$expr": { "$eq": [ "$_id", "ROOT_DOCUMENT_ID" ] } } },
    {
        $project: {
            "_id": 0,
            "comments": {
                $let: {
                    "vars": {"array1": "$$ROOT.array1" },
                    "in": {
                    $let: {
                        "vars": {
                        "array1_member": {
                            $arrayElemAt: [
                            {
                                $filter: {
                                "input": "$$array1",
                                "as": "m",
                                "cond": {
                                    $eq: [
                                    "$$m._id",
                                    "ARRAY1_B"
                                    ]
                                }
                                }
                            },
                            0
                            ]
                        }
                        },
                        "in": {
                        $let: {
                            "vars": {
                            "array2_member": "$$array1_member.array2"
                            },
                            "in": {
                            $let: {
                                "vars": {
                                "array2": {
                                    $arrayElemAt: [
                                    "$$array2",
                                    0 // This is the index I want
                                    ]
                                }
                                },</pre>
                                "in": "$$array2_member.comments"
                            }
                            }
                        }
                        }
                    }
                    }
                }
            }
        }
    }
])

This gives the desired output.

For the second query, I was trying to work with that but I don’t see array1 or array2 being referenced anywhere or matched. I don’t really see how to fit that into my case.

With the following data:

{
    "_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
                }
            ]
        }
    ]
}

The only information I have is: _id (root document ID), array1._id, and the index of array 2.

Appreciate the help again!