MongoDB: Updating element in array always results in first element updated

Question

TL;DR : Updating element in array always results in first element updated and results differ based on property name of key used in find params Playground : https://mongoplayground.net/p/-4kGZnxa-WA

I want to update an object in a array and I am using 2 of the object properties to find the object then using $set operator with array.$.updateProperty to update the object

Here is the working playground link of what I want to do: https://mongoplayground.net/p/dswt8vuzJMc

But I cant reproduce the same when I change a single property name (both in database as well as find parameter) , from the above example I changed property foo to trackID but then only the first element in array is always updated

Playground : https://mongoplayground.net/p/-4kGZnxa-WA

It seems weird as I assumed the property name shouldn't matter as long as it used the same in find params too and its not a keyword like _id

rickhg12hs · Accepted Answer · 2022-08-11 23:14:52Z

2

Your update is very close. You need to use "$elemMatch" to identify the specific array position where both conditions match.

N.B.: $ will only update the first matching array element. If you want to update all array elements, use $[], and if you want to update all matching array elements, using "arrayFilters" with $[<indentifier>] is convenient.

db.collection.update({
  "_id": ObjectId("62f11e22d99c79532de6ff7f"),
  "jobs": {
    "$elemMatch": {
      "trackID": 0,
      "name": "kaisen_track-0_h264_1080p"
    }
  }
},
{
  "$set": {
    "jobs.$.status": "Done"
  }
})

Try it on mongoplayground.net.

edited Aug 11, 2022 at 23:14

answered Aug 11, 2022 at 22:44

rickhg12hs

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3 Comments

Solaris Over a year ago

Thanks , works perfect but I am still confused why did the operation worked on this playground (working playground mentioned in question): mongoplayground.net/p/dswt8vuzJMc The only difference I see with both the playgrounds in question is the property name , am I missing any other difference ?

rickhg12hs Over a year ago

@Solaris I'm a little confused too. I don't know the internals of mongod so I can only speculate. By some means (?), in your original query ,mongod determines the array element of each match independently (?) and if they both happen to be the same, then $ works as a valid array position? It's actually a little scary since if the mongoplayground prototyping worked (by some lucky/unlucky occurrence), there's no guarantee (unlikely really) that the luck would carry over to your real/production system. That's why learning the proper query syntax, here "$elemMatch", is so important.

Solaris Over a year ago

Might be a bug or unhandled checks on necessary syntax , anyhow the proper syntax $elemMatch is working consistently as expected so will stick to it, thanks for the help

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