Mongodb nested array update query

Question

This is a sample collection in which I want to update a user's posts. In fact, I want to operate a command similar to $push and $pop so that I can remove or add the userId from the likes array inside the posts array, this seems fairly simple but the catch is that I want to update the likes of a particular post matched by _id field of objects within posts array.

[
  {
    "_id": {
      "$oid": "61375acc1c7d0a1a6e6005f0"
    },
    "fullName": "user1",
    "email": "[email protected]",
    "password": "$2b$10$Yrs5H3mYrM8xLwWlek3K7uAs.EOLsXggj6wV7oSflPlPjo1ZkFem6",
    "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
    "posts": [
      {
        "postContent": "sample 1",
        "medias": [
          "files/78560be22a25988c38ddafa0be7558f73713607a.jpeg"
        ],
        "createdAt": {
          "$date": "2021-09-07T12:40:54.930Z"
        },
        "userId": {
          "$oid": "61375acc1c7d0a1a6e6005f0"
        },
        "authorName": "user1",
        "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
        "likes": [],
        "comments": [],
        "_id": {
          "$oid": "61375dd61c7d0a1a6e6005f3"
        }
      },
      {
        "postContent": "sample 2",
        "medias": [
          "files/5af7e5086e8dfb8af6c8e7bac3f2430a78d80ac1.jpeg",
          "files/c01ed66cef55c876c8e7800aa5c820b9aeee7267.jpeg"
        ],
        "createdAt": {
          "$date": "2021-09-07T12:42:01.142Z"
        },
        "userId": {
          "$oid": "61375acc1c7d0a1a6e6005f0"
        },
        "authorName": "user 2",
        "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
        "likes": [],
        "comments": [],
        "_id": {
          "$oid": "61375e191c7d0a1a6e6005f4"
        }
      }
    ],
    "friendRequests": []
  }
]

i.e, once a query is executed the likes array of the particular post (matched by _id) has to be updated with the userId (both userId and postId will be input in this case)

eg:

document.posts.likes =  [61375acc1c7d0a1a6e6005f0, 61375acc1c7d0a1aasdfasgre3]

No read the last part... Likes array inside posts array objects should store id of users who liked the respective posts — Vishnu Mohan
– Vishnu Mohan, Commented Sep 7, 2021 at 14:40

turivishal · Accepted Answer · 2021-09-07 14:41:22Z

There is no straight way to do this, you can try update with aggregation pipeline starting from MongoDB 4.2,

$map to iterate look of posts array
$cond to check if post id match then go to update part otherwise return same object
$cond to check if user id in likes array then go to remove part otehrwise add id in likes
$filter to iterate loop of likes array and remove user id
$concatArrays add user id in likes array
$mergeObjects to merge current object with updated likes array field

let postId = { "$oid": "61375e191c7d0a1a6e6005f4" };
let userId = "61375dd61c7d0a1a6e6005f1";
db.collection.updateOne(
  { "posts._id": postId },
  [{
    "$set": {
      "posts": {
        $map: {
          input: "$posts",
          as: "p",
          in: {
            $cond: [
              { $eq: ["$$p._id", postId ] },
              {
                $mergeObjects: [
                  "$$p",
                  {
                    "likes": {
                      "$cond": [
                        { $in: [userId, "$$p.likes"] },
                        {
                          $filter: {
                            input: "$$p.likes",
                            cond: { $ne: ["$$this", userId] }
                          }
                        },
                        {
                          "$concatArrays": ["$$p.likes", [userId]]
                        }
                      ]
                    }
                  }
                ]
              },
              "$$p"
            ]
          }
        }
      }
    }
  }]
)

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