Query multiple indices in MongoDB array

Question

I want to query some specific non-consecutive indexes of an array in a MongoDB document.
The options I have considered are:

Query the entire array, then filter by index in client script. This isn't viable as for a large array it will result in fetching unwanted items from the database (Suppose I need only index 5 and 20, but there are actually 1000 items in the array).
Using $slice multiple times picking every time each of the indices I want from the array. But this results in N number of calls.
Instead of a simple array, I can use a child schema where every item in the array has an assigned Id, and while querying, I can use "find" to select those specific ids. But this adds an extra "id" field which I do not want. I am sure there must be a way to pick items by the array index.

I am looking for a more elegant solution where I can pick certain specific indices from the array in a single operation.

Example document in Quiz model:

{
     questions: [
          {
               text: "Question 1",
               answer: "Answer 1",
          },
          {
               text: "Question 2",
               answer: "Answer 2",
          },
          ...
          ...
          {
               text: "Question 1000",
               answer: "Answer 1000",
          },
     ]
}

I want something in the lines of:

const questions = await Quiz.find({
     questions: {
          $index_in: [10, 42, 66]
     }
}).exec();

Expected output:

[
     {
          text: "Question 10",
          answer: "Answer 10",
     },
     {
          text: "Question 42",
          answer: "Answer 42",
     },
     {
          text: "Question 66",
          answer: "Answer 66",
     },

Are you trying to select a consecutive set of items? If so, a query, projection, then limit / offset should do it. If you are selecting several items at non-consecutive indexs I'm curious how you know the indexs in the first place? — robinsax
– robinsax, Commented Aug 21, 2021 at 21:40
@robinsax - They are non-consecutive indexes. In an earlier operation, the code picks a few random indices (random questions for the quiz example I added) from the array of items, at which point we have the index numbers. At a later stage, client "submits" those questions back, which contains the index information. Now I need to query the particular index items again, to match the user submitted answer with the database. Hope I could explain. — Chique
– Chique, Commented Aug 21, 2021 at 22:07

Takis · Accepted Answer · 2021-08-22 02:17:31Z

3

If you want to take specific indexes from an array

Test code here

Query

Replace

[0,2,5] with the indexes you want (or a driver variable that is array)
instead of "$myarray", put your field name (in your case "$questions")

Its fast because $arrayElemAt is O(1)

db.collection.aggregate([
  {
    "$addFields": {
      "myarray": {
        "$map": {
          "input": [
            0,
            2,
            5
          ],
          "as": "i",
          "in": {
            "$arrayElemAt": [
              "$myarray",
              "$$i"
            ]
          }
        }
      }
    }
  }
])

answered Aug 22, 2021 at 2:17

Takis

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1 Comment

Chique Over a year ago

This works exactly as I was expecting. Also this is much faster than all the other options I have mentioned in my question, by more than 50%. Thank you so much.

NeNaD · Accepted Answer · 2021-08-21 22:16:04Z

0

You can do it with aggregation pipeline, like this:

const questions = await Quiz.aggregate([
  {
    $project: {
      items: {
        $filter: {
          input: "$questions",
          cond: {
            $in: [
              "$$this.text",
              [
                "Question 2",
                "Question 5"
              ]
            ]
          }
        }
      }
    }
  }
]);

Now, you can just dynamically populate wanted questions.

Here is the working example: https://mongoplayground.net/p/aXRiVSawpLf

answered Aug 21, 2021 at 22:16

NeNaD

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1 Comment

Chique Over a year ago

I can't search by text as the texts may vary. I specifically need to search by index positions.

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