How to do several searches in the database at the same time (nodejs, mongodb)?

Question

MongoDB

There is a collection that stores comments, one of the comment fields is the id of the post to which it refers. How to make several finds so that you can display the number of comments for each post at a time (there will be several posts on the page at once)? And it is also unknown how many posts will be on the page. In general, everything should be like this: the script receives an array of post ids, makes queries to the database and returns an object of similar content:

{
  "10": 13,
  "11": 5,
  "12": 27
}

Or can it be done in a more concise way?

Answer 1

You need to use $group :

> db.comments.insert({id_post: 1})
> db.comments.insert({id_post: 1})
> db.comments.insert({id_post: 1})
> db.comments.insert({id_post: 2})
> db.comments.insert({id_post: 2})
> db.comments.insert({id_post: 3})
> db.comments.aggregate([{$group: {_id: "$id_post", count: {$sum: 1}}}])
{ "_id" : 3, "count" : 1 }
{ "_id" : 2, "count" : 2 }
{ "_id" : 1, "count" : 3 }