MongoDB update array element of a document by index in nodeJS

Question

How can I increment the value of an specific element of an array of a MongoDB document in Meteor or nodeJS?

Say, I've following document:

{
    "_id" : "4tP6ewe4Z5kwYA3Je",
    "name" : "chutia",
    "address" : "shonir akhra",
    "itemCount" : "4",
    "items" : [ 
        "3", 
        "4",
        "13", 
        "24"
    ]
}

I need to increment the n'th element of items array. Where n is a variable.

Community · Accepted Answer · 2017-05-23 11:53:20Z

2

The right way to do this is:

{$inc: {[`items.${idx}`]: 1}}

Where idx is the array index. Courtesy MasterAM's comment

edited May 23, 2017 at 11:53

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answered Feb 15, 2017 at 7:31

Mostafiz Rahman

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2 Comments

user1063287 Over a year ago

does this type of syntax have a name (or is the convention documented somewhere?) - ie the array containing a template string - i've never seen it before, it took me a while to figure out what i was looking at! i still don't understand how/why the "array brackets" make the key name acceptable. (it did work for me though!)

user1063287 Over a year ago

update to earlier comment if it helps anyone else - it seems it is a "computed key", see: stackoverflow.com/a/30969495 and stackoverflow.com/a/2274327 and stackoverflow.com/a/19837961

legendecas · Accepted Answer · 2017-02-14 10:32:29Z

1

As documented here https://docs.mongodb.com/manual/reference/operator/update/inc/, you can increment an element in an array by dot notation.

The following example increase the first element of items by 1.

db.<collection>.update(
 <query>,
 { $inc: { "items.0": 1 } }
)

answered Feb 14, 2017 at 10:32

legendecas

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2 Comments

Mostafiz Rahman Over a year ago

In my case the index of the array will be a variable. So, "items.0" won't work I guess. How to deal with this?

MasterAM Over a year ago

{$inc: {[`items.${idx}`]: 1}}, where idx is your index.

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