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.

score 2 · Accepted Answer · edited May 23 '17 at 11:53

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 '17 at 11:53

Community

1
1

answered Feb 15 '17 at 07:31

Mostafiz Rahman

8,169
7
57
74

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 Jun 22 '19 at 11:09
update to earlier comment if it helps anyone else - it seems it is a "computed key", see: https://stackoverflow.com/a/30969495 and https://stackoverflow.com/a/2274327 and https://stackoverflow.com/a/19837961 – user1063287 Jun 22 '19 at 11:15

score 1 · Answer 2 · answered Feb 14 '17 at 10:32

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 '17 at 10:32

legendecas

359
2
8

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? – Mostafiz Rahman Feb 14 '17 at 10:45
6

```{$inc: {[`items.${idx}`]: 1}}```, where `idx` is your index. – MasterAM Feb 14 '17 at 12:13

MongoDB update array element of a document by index in nodeJS

2 Answers2