Mongodb how to count how many times a field value comes?

Question

let´s say I have a document of blog posts that has fields "_id, userName, age". a user could have made more than one blog post, I want to find the users that have made 4 posts. db.blogs.aggregate([{$group: {_id: {"$userName", "age"}, : {$sum: ""eq", 3}}])

Answer 1

To know how many times a field comes, use a $group stage to group by that field and add an extra field for the count using the $count operator. Then if you want to filter by that count, just add a $match stage to filter by that new field:

{
  $group: {
    "_id": "$username",
    "count": {
      $count: {}
    }
  }
},
{
  $match: {
    "count": {
      $gte: 4
    }
  }
}

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