See if nested array of objects contains value | MongoDB

Question

I have a collection posts which has an array of objects comments. Within this array of objects, I have another array of objects likes.

I am trying to write a query that pulls the most recent 5 comments from a post, and only pulls true or false for likes, based on if the user has liked the comment already.

I have written this so far:

db.posts.aggregate([
    {
        "$match": {
            "_id": postId
        }
    },
    {
        "$project": 
        {
            "comments": {
                "$slice": [ "$comments", -5 ]
            }
        }
    },
    {
        "$project": {
            "comments.content": 1,
            "comments.likes": { 
                "$eq":  [ "comments.likes.$.createdBy.username", username ] 
            } 
        }
    }
])

But this seems to pull false everytime.

Is it possible to do this without having to write a separate query to check if the user has liked?

EDIT: So for the below document:

With username = "testusername", and postId = "60fcd335abbe5a73583b69f0"

I would expect output:

[
    {
        "content": "test comment",
        "likes": true
    },
    {
        "content": "another test comment",
        "likes": true
    }
]

And with username = "testusername2" I would expect output

[
    {
        "content": "test comment",
        "likes": true
    },
    {
        "content": "another test comment",
        "likes": false
    }
]

Answer

Thanks to @ray for your help with this one. Condensed Code Here though please see ray's response for the code split out with explanation.

would be helpful if you can provide sample data and expected output for everyone here to take a look — ray
– ray, Commented Jul 25, 2021 at 2:56
another piece of advice: paste the code/sample data in text instead of image; It would be easier for others to copy and replicate it — ray
– ray, Commented Jul 25, 2021 at 4:02

ray · Accepted Answer · 2024-04-13 18:05:22Z

1

You can use $map to process your arrays layer-by-layer.

You can first $map comment to project a boolean for if the likes are liked by user1
Then you can use $anyElementTrue to perform the checking on the projected boolean

db.posts.aggregate([
  {
    "$match": {
      "_id": "p1"
    }
  },
  {
    "$project": {
      "comments": {
        "$slice": [
          "$comments",
          -5
        ]
      }
    }
  },
  {
    "$project": {
      likes: {
        "$map": {
          "input": "$comments",
          "as": "c",
          "in": {
            content: "$$c.content",
            likedByU1: {
              "$map": {
                "input": "$$c.likes",
                "as": "l",
                "in": {
                  $eq: [
                    "$$l.createdBy._id",
                    "u1"
                  ]
                }
              }
            }
          }
        }
      }
    }
  },
  {
    "$project": {
      likes: {
        "$map": {
          "input": "$likes",
          "as": "l",
          "in": {
            content: "$$l.content",
            likedByU1: {
              "$anyElementTrue": [
                "$$l.likedByU1"
              ]
            }
          }
        }
      }
    }
  }
])

Here is the Mongo playground to show the idea(with some minor modifications to your example). You can modify it to fit your needs.

edited Apr 13, 2024 at 18:05

answered Jul 25, 2021 at 3:54

ray

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3 Comments

bourkison Over a year ago

This is perfect. Haven't used $map much before so thank you for this, and for bringing Mongo playground to my life.

bourkison Over a year ago

FYI have played around with it a bit to avoid a second projection: Mongo Playground

ray Over a year ago

@bourkison your new code is cleaner. The reason I split the code into 2 stages is for demonstration purpose so you can run the stages separately for better understanding. I would suggest you supplement your new version in your question to help more people out :)

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