How to pass the index of an array element parametrically in MongoDB?

Question

I have a collection of documents similar to the following:

{
    _id: ObjectId("..."),
    title: "...",
    body: "...",
    comments: [
        {
            name: "...",
            email: "...",
            comment: "..."
        },
        {
            name: "...",
            email: "...",
            comment: "..."
        },
        {
            name: "...",
            email: "...",
            comment: "..."
        }
    ]
}

Let's say that I want to update the name field for the second comment. So, I would have a query as follows:

db.posts.update({_id: ""}, {$set: {"comments.2.name": "new name"}});

Now, I'm wondering how could I pass the index of array element (2) as a parameter/variable in node.js?

Blakes Seven · Accepted Answer · 2015-07-13 00:43:43Z

2

The answer to this is to simply construct the object in code. Object keys stringify in JavaScript and most other languages supporting the notation, so simply build the string externally and use the Object notaion methods:

var index = 1,  // n-1 for array index notation of the second element
    name = "new name";


var update = { "$set": { } },
    query = {};

update["$set"]["comments." + index + ".name"] = name;

// Then update as normal
db.posts.udate(query,update);

That builds an object using the correct "dot notation" format

answered Jul 13, 2015 at 0:43

Blakes Seven

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2 Comments

Blakes Seven Over a year ago

@moorara Of course it works, it's standard JavaScript notation. You are probably doing something like this: { "$set": { "comments." + index + ".name": name } }, and that is incorrect and does not work. The MongoDB shell is a REPL. Just type it in any you will clearly see the object is formed correctly.

Gobliins Over a year ago

how can you do this inside an update hook when you have the $set modifier: modifier.$set.myArray.index.value where index is the position of the element to update and value is what i want to set?

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