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.

Answer 1

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

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

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