MongoDB aggregation query - return all items inside nested subdocument

Question

Basically, I have a nested array of objects. I want to extract information from those objects, and return all the results as an array.

Here is my data:

[
  {
    "_id": "608642db80a36336946620aa",
    "title": "titleHere",
    "types": {
      "flashcardReversed": [
        {
          "normal": { // the data inside of "normal" should become its own object in the returned array
            "_id": "608d5b290e635ece6828141X",
            "front": "2front",
            "back": "2back"
          },
          "reversed": { // the data inside of "reversed" should become its own object in the returned array
            "_id": "608t5b290e635ece6828141Y",
            "front": "2frontReversed",
            "back": "2backReversed"
          }
        },
        {
          "normal": {
            "_id": "608a5b31a3f9806de253726X",
            "front": "2front2",
            "back": "2back2"
          },
          "reversed": {
            "_id": "608a5b31a3f9806de253726Y",
            "front": "2frontReversed2",
            "back": "2backReversed2"
          }
        }
      ]
    }
  }
]

Here is the desired result:

[
  {
    "flashcardReversed": [
      {
        "_id": "608d5b290e635ece6828141X",
        "front": "2front",
        "back": "2back"
      },
      {
        "_id": "608t5b290e635ece6828141Y",
        "front": "2frontReversed",
        "back": "2backReversed"
      },
      {
        "_id": "608t5b290e635ece6828141Y",
        "front": "2frontReversed",
        "back": "2backReversed"
      },
      {
        "_id": "608a5b31a3f9806de253726Y",
        "front": "2frontReversed2",
        "back": "2backReversed2"
      }
    ]
  }
]

My current attempt: https://mongoplayground.net/p/k3Fjd59KMKg I have tried merging the $card and $card2 documents through various methods, and then using $replaceRoot to create the desired result - but after much time trying, I was unsuccessful.

Some notes about my use-case - I will be using this pipeline inside $facet There will be many objects inside of flashcardReversed, all created from the same Mongoose Schema

The document should have a max of ~2000 cards. I need the results for use in a react website. Should I be worried about the efficiency of this search? (if this should be a question in itself I would be happy to move it).

If my Mongoose Schemas would be helpful, let me know and I can add them.

All help is appreciated,

- Riley Swinson

Is it always "normal" and "reversed" keys in the original? Or can there be more ? — Cody Geisler
– Cody Geisler, Commented May 5, 2021 at 1:47
@CodyＧ there can be many documents, all with the same structure. I'll clarify my post. Thanks! — Sour_Tooth
– Sour_Tooth, Commented May 5, 2021 at 1:47

susheem_k · Accepted Answer · 2021-05-05 03:34:38Z

I was able to come up with this query to get the desired result

    db.collection.aggregate([
  {
    "$unwind": "$types.flashcardReversed"
  },
  {
    "$project": {
      "flashcardReversed": {
        "$map": {
          "input": {
            "$objectToArray": "$types.flashcardReversed"
          },
          "as": "elem",
          "in": "$$elem.v"
        }
      }
    }
  },
  {
    "$unwind": "$flashcardReversed"
  },
  {
    "$group": {
      "_id": 1,
      "flashcardReversed": {
        "$push": "$flashcardReversed"
      }
    }
  },
  {
    "$project": {
      "_id": 0
    }
  }
])

Explanation of the above pipeline :

Use $unwind to divide the array into its elements
Use $project along with $map to convert the flashcardReversed object into an array of values. Using $map allows the pipeline to handle any additional keys on top of normal and reversed
Use $unwind to again split the flashcardReversed array formed in step 2
Use $group with the $push accumulator to add all values of flashcardReversed in a single array
Use $project to remove the _id field from the response

Here's a Mongodb playground link for the same

Joe · Accepted Answer · 2021-05-05 03:08:05Z

1

This can be done with

$unwind to split the flash card array
$project to convert the object containing normal/reverse to an array
$unwind the created array
$group by _id and collect the values in an array

db.collection.aggregate([
  {$unwind: "$types.flashcardReversed"},
  {$project: {
      flashcardReversed: [
        "$types.flashcardReversed.normal",
        "$types.flashcardReversed.reversed"
      ]
  }},
  {$unwind: "$flashcardReversed"},
  {$group: {
      _id: "$_id",
      flashcardReversed: {$push: "$flashcardReversed"}
  }}
])

answered May 5, 2021 at 3:08

Joe

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7 Comments

Sour_Tooth Over a year ago

Thanks for answering - I appreciate it. I noticed an error in my question - because I am using $facet, I already have"flashcardReversed" as the parent: []. How would I return just what is inside of the flashcardReversed array in your result?

Joe Over a year ago

Use a $replaceRoot stage

Sour_Tooth Over a year ago

tried, but got error $replaceRoot must be of type object. Attempted to use $arrayToObject, but I think I need a different object structure for that to work?

Joe Over a year ago

{$replaceRoot:{newRoot:"$flashcardReversed"}}

Sour_Tooth Over a year ago

Yes, exactly what I tried, and still returns an error - mongoplayground.net/p/CiaOOrzzw5i. The goal would be to remove all the contents from the array - since $facet is already an array this seems like it could be a solution

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