Updating an element with a given field within a tree-like structure in mongodb

Question

For example, when I have a comment structure looking like that within a single collection entry:

"comments": [
    {
        "id": 0,
        "comments": []
    },
    {
        "id": 1,
        "comments": [
            {
                "id": 3,
                "comments": [
                    {
                        "id": 5,
                        "comments": []
                    },
                    {
                        "id": 6,
                        "comments": []
                    }
                ]
            }
        ]
    },
    {
        "id": 2,
        "comments": [
            {
                "id": 4,
                "comments": []
            }
        ]
    }
]

Is there a nice way to push another comment to a comment with a given id value?

Answer 1

This structure is quite complex to query/update dynamically due to the recursive nature of the schema.

There is no "easy" way to achieve what you're asking for without knowing the "depth" of the message, What I recommend if you want to keep the current structure is to add a new field to each message called "depth", by knowing this ahead of time it will be easier to generate a query on the fly to update the right message, like so:

const depth = 2
const id = 5

const queryKey = `${Array(depth + 1).join('comments.')}id`;
const query = {
    [queryKey]: id
};
const updateKey = `${Array(depth + 1).join('comments.$')}[elem].comments`;

db.collection.updateOne(
    query,
    {
      "$push": {
        [updateKey]: {
          id: "newId",
          depth: depth + 1,
          comments: []
        }
      }
    },
    {
      arrayFilters: [
        {
          "elem.id": id
        }
      ]
    })

This will dynamically generate the proper query to update the correct subdocument.

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