Update inner array's element in array of arrays in mongodb

Question

I have the following mongodb document structure:

{
    "_id" : "c6b8S5Rx8xZiL42WS",
    "rows" : [
                [
                        {
                                "row" : 1,
                                "col" : 1,
                                "reserved" : false
                        },
                        {
                                "row" : 1,
                                "col" : 2,
                                "reserved" : false
                        },
                ],
                [
                        {
                                "row" : 2,
                                "col" : 1,
                                "reserved" : true
                        },
                        {
                                "row" : 2,
                                "col" : 2,
                                "reserved" : false
                        },
                ],
            ]
}

Is it possible to somehow update for example the inner object where row=1 and col=1 in this document?
Or should I just change the whole document's structure/maybe create a new collection?

It's Mongo 3.4.5 which I unfortunately can't update because I'm using a framework called Meteor.js that uses it's own supported version of mongo — Relicho
– Relicho, Commented Feb 21, 2018 at 17:58

s7vr · Accepted Answer · 2018-02-22 00:19:48Z

2

You can try the below update.

Note the use of double $elemMatch to reach the nested array and use positional operator with $set and index variable to update the inner array element.

Positional operator returns the index of outer array.

db.col.update(
  {
    "_id": "c6b8S5Rx8xZiL42WS",
    "rows": {
      "$elemMatch": {
        "$elemMatch": {
          "row": 1,
          "col": 1
        }
      }
    }
  },
 { "$set": { ['rows.$.' + index + '.reserved']: true } } 
)

edited Feb 22, 2018 at 0:19

answered Feb 21, 2018 at 18:07

s7vr

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8 Comments

Relicho Over a year ago

For some reason it matches the document but it doesn't update it for me, it says: { "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 }

s7vr Over a year ago

Which query did you use ? Use first one

Relicho Over a year ago

I used the first one but I think I did something wrong. Here's what I'd like to achieve: I want to update the inner object's reserved field to true where row=1, col=1 without modifying the structure of the document. How should I modify your query?

s7vr Over a year ago

Sorry I mixed up your question with some other question. Deep level updates is not possible in 3.4 but you can try something like

db.col.update(   {     "_id": "c6b8S5Rx8xZiL42WS",     "rows": {       "$elemMatch": {         "$elemMatch": {           "row": 1,           "col": 1         }       }     }   },   {     "$set": {       "rows.$.0.reserved": true     }   } )

. Note I'm using 0 index to refer to first element inside inner array. So if you have that kind of flexibility it may work for your use case or else you have model the collection in a more update friendly way.

Relicho Over a year ago

This would work for me but I would have to make the "0" in the "rows.$.0.reserved" to be dynamic based on a parameter because I'm making this update from a function. Is that possible?

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