MongoDB: Query single document from multiple embedded arrays?

Question

A have a collection with objects each with an array of documents, I want to query for a specific single documents across all the arrays.

Currently to do this I am finding the document with the array which contains the document I'm looking for, then projecting for this specific document.

This approach seems like an inefficient work around because:

It will always iterate across all values without the ability to exit early when the desired document is found.
It seems as if it would be O(2n) since it iterates across all values for the match then all values again for the projection.
Returned document is awkwardly formed.

Is there a better approach for this?

Live example: https://mongoplayground.net/p/WowuB-mZpPI

db.collection.aggregate([
  {
    "$match": {
      "values.name": "delta"
    }
  },
  {
    "$project": {
      "value": {
        "$filter": {
          "input": "$values",
          "cond": {
            "$eq": [
              "$$this.name",
              "delta"
            ]
          }
        }
      }
    }
  }
])

Example collection:

[
  {
    "group_name": "first",
    "members": [
      {
        "name": "alpha",
        "value": 1
      },
      {
        "name": "beta",
        "value": 2
      },
      {
        "name": "gamma",
        "value": 3
      },
      
    ]
  },
  {
    "group_name": "second",
    "values": [
      {
        "name": "delta",
        "value": 4
      },
      {
        "name": "epsilon",
        "value": 5
      },
      {
        "name": "zeta",
        "value": 6
      },
      
    ]
  }
]

Jonathan Woollett-light · Accepted Answer · 2021-07-10 00:15:12Z

If you don't have an index on "values.name", then searching k documents with n elements per array would be O(kⁿ).

If you do have that index, mongodb will be able to scan the index for only matching values, so it would be dependent on the number of documents that have an element that matches rather than the number of documents or elements.

The $filter itself is intended to return an array of all elements that match, so it would need to check every element.

If you want to shortcut that, you would need to use operators that only match one element, perhaps $indexOfArray paired with $arrayElemAt.

https://mongoplayground.net/p/RP634lXWaVU

db.collection.aggregate([
  {
    "$match": {
      "values.name": "delta"
    }
  },
  {
    "$project": {
      "value": {
        "$arrayElemAt": [
          "$values",
          {
            "$indexOfArray": [
              "$values.name",
              "delta"
            ]
          }
        ]
      }
    }
  }
])

Collectives™ on Stack Overflow

MongoDB: Query single document from multiple embedded arrays?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related