Mongodb Retrieve first element that match condition in array

Question

I've tried to read around before post this question but wasn't able to figure this out.

I'm trying to return only the first element that matches a given condition from a query in my database.

The collection documents are like this:

{
    "key": 1,
    timestamps: {
      history: [
        {
          time: 10
        },
        {
          time: 9
        },
        {
          time: 8
        },
        
      ]
    }
  },

And I want to retrieve for example the first element inside the history array with a timestamp lower than 10.

So the expected result should be something like this:

[
  {
    "timestamps": {
      "history": [
        {
          "time": 9
        }
      ]
    }
  }
]

You can find an example into this playground https://mongoplayground.net/p/jHKb9o-0tHm .

So far I'm able to retrieve all the elements inside the array that are lower than 10 but not only the first match, how can I accomplish that?

Also since I'm trying to make it as efficient as possible, is it faster to retrieve only the first element for the db or it is only an additional operation to do for the database because the operation of returning only first element is done after that the db already found all the elements?

You can use the $elemMatch (projection) which is simpler and concise rather than the aggregate $filter. — prasad_
– prasad_, Commented Dec 29, 2021 at 4:59
@prasad_ thanks,i tried as you sudjested mongoplayground.net/p/iXUDD31cDqO but unfortunately it says that i cannot use $elemMatch on nested arrays — Linch1
– Linch1, Commented Dec 29, 2021 at 9:26

prasad_ · Accepted Answer · 2021-12-29 10:40:01Z

1

The array field "timestamps.history" is a nested field and to get the first matching element using $elemMatch projection operator fails with an error: "Cannot use $elemMatch projection on a nested field.".

In this case use the Positional Projection Operator $ as shown below to get the desired result.

db.collection.find(
 {
   key: 1, "timestamps.history.time": { $lt: 10 }
 },
 {
   "timestamps.history.$": 1
 }
)

answered Dec 29, 2021 at 10:40

prasad_

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2 Comments

Linch1 Over a year ago

Thanks that works, do you also know how can i sort the elements before doing the projection ? I mean in this sandbox mongoplayground.net/p/lIaBXRSGSTN i'd like the query to return the history with time: 9 instead of time: 8. Or maybe getting the last element instead the first one, i tried to put as index -1 in the projection but does not work

prasad_ Over a year ago

The projection picks the first matching element. You can use an aggregation query for more specific matching or projection.

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