MongoDB - get all elements matching a condition in "deep" array

Question

My users collection contains:

{
    _id: "1",
    posts: [
        {
            _id: "12",
            comments: [
                {
                    _id: "123",
                    userId: "5",
                    ...
                },
                ...
            ]
        },
        ...
    ]
},
...

I want to get all the comments the belongs to specific user id (userId prop).

How can I achieve that?

Thanks

Mohammed Yousry · Accepted Answer · 2020-05-06T19:34:39.107

0

you can try this

db.collection.aggregate([
  {
    $match: {
      "posts.comments.user": "userId1" // note that you need to use objectId here if the user property in the comments array is an objectId
    }
  },
  {
    $unwind: "$posts"
  },
  {
    $project: {
      "posts.comments": {
        $filter: {
          input: "$posts.comments",
          as: "comment",
          cond: {
            $eq: [
              "$$comment.user",
              "userId1"
            ]
          }
        }
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      posts: {
        $push: "$posts"
      }
    }
  }
])

test it here Mongo Playground

> Update

the above query will get an array of documents, each document has posts array, and each post element has comments array

if you need just the comments array belong to some user, then we can use something like this

db.collection.aggregate([
  {
    $match: {
      "posts.comments.user": "userId1"
    }
  },
  {
    $unwind: "$posts"
  },
  {
    $project: {
      "posts.comments": {
        $filter: {
          input: "$posts.comments",
          as: "comment",
          cond: {
            $eq: [
              "$$comment.user",
              "userId1"
            ]
          }
        }
      }
    }
  },
  {
    $unwind: "$posts.comments"
  },
  {
    $group: {
      _id: null,
      comments: {
        $push: "$posts.comments"
      }
    }
  },
  {
    $project: {
      _id: 0
    }
  }
])

and test it here Mongo Playground 2

edited May 06 '20 at 19:34

answered May 06 '20 at 12:34

Mohammed Yousry

2,134
1
5
7

Is it efficient to first unwind the posts property of all users? Isn't it will be better to first match users that contains offers with the given userId? – user3343396 May 06 '20 at 12:52
yes of course, I've updated my answer, could you check it now? – Mohammed Yousry May 06 '20 at 12:57
note that you need to use ObjectId in $match if the user property is of type ObjectId – Mohammed Yousry May 06 '20 at 13:00
It works well! But in the end i need an array of comments, should i do it in Javascript and not in the aggregation? – user3343396 May 06 '20 at 13:19
you need an array of comments directly, without posts then comments inside it? – Mohammed Yousry May 06 '20 at 13:20
Yes, how can I achieve this? – user3343396 May 06 '20 at 17:43
could you try this https://mongoplayground.net/p/m9pb3K3sgDq, If that's okay, just let me know to update the answer – Mohammed Yousry May 06 '20 at 19:03

MongoDB - get all elements matching a condition in "deep" array

1 Answers1

> Update