How to do HAVING COUNT in MongoDB?

Question

My documents look like this:

{
    "_id": ObjectId("5698fcb5585b2de0120eba31"),
    "id": "26125242313",
    "parent_id": "26125241841",
    "link_id": "10024080",
    "name": "26125242313",
    "author": "gigaquack",
    "body": "blogging = creative writing",
    "subreddit_id": "6",
    "subreddit": "reddit.com",
    "score": "27",
    "created_utc": "2007-10-22 18:39:31"
}

What I'm trying to do is create a query that finds users who posted to only 1 subreddit. I did this in SQL by using the query:

Select distinct author, subreddit from reddit group by author having count(*) = 1;

I'm trying to do something similar in MongoDB but are having some troubles atm. I managed to recreate select distinct by using aggregate group but I can't figure out how to solve the HAVING COUNT part.

This is what my query looks like:

db.collection.aggregate( 
[{"$group": 
    { "_id": { author: "$author", subreddit: "$subreddit" } } },
    {$match:{count:1}} // This part is not working
])

Am I using $match wrong?

Answer 1

Your query should be like:

db.collection.aggregate([{
  '$group': {
    '_id': {'author': '$author', 'subreddit': '$subreddit'}, 
    'count': {'$sum': 1}, 
    'data': {'$addToSet': '$$ROOT'}}
}, {
  '$match': {
    'count': {'$eq': 1}
}}])

Where data is one-length list with matched document.

if you want to get some exact field, it should look like this:

db.collection.aggregate([{
  '$group': {
    '_id': {'author': '$author', 'subreddit': '$subreddit'}, 
    'count': {'$sum': 1}, 
    'author': {'$last': '$author'}}
}, {
  '$match': {
    'count': {'$eq': 1}
}}])