Filtering a list of votes where more than x matches are found - mongodb

Question

I have the following vote data in a large collection:
{
"user_id" : ObjectId("53ac7bce4eaf6de4d5601c1a"),
"article_id" : ObjectId("53ab27504eaf6de4d5601be5"),
"score" : 5
},
{
"user_id" : ObjectId("53ac7bce4eaf6de4d5601c1b"),
"article_id" : ObjectId("53ab27504eaf6de4d5601be5"),
"score" : 3
},
{
"user_id" : ObjectId("53ac7bce4eaf6de4d5601c1c"),
"article_id" : ObjectId("53ab27504eaf6de4d5601be5"),
"score" : 3
},
...
I'm looking to filter this collection where more than 3 votes have been obtained for a single article (as above) and output as-is (excluding any vote entries on articles < 3 total votes).
Any help much appreciated. This collection can be huge so efficiency would be ideal.

Answer 1

Normally not something you do in a single operation, but you can do this if those really are your only fields and there are not too many matching documents.
db.collection.aggregate([
{ "$group": {
"_id": "$article_id",
"docs": {
"$push": {
"user_id": "$user_id",
"article_id": "$article_id",
"score": "$score"
}
},
"votes": { "$sum": 1 }
}},
{ "$match": { "votes": { "$gt": 3 } } },
{ "$unwind": "$docs" },
{ "$project": {
"user_id": "$docs.user_id",
"article_id": "$docs.article_id",
"score": "$docs.score"
}}
])
You can clean that up a little with MongoDB 2.6 and greater which provides a system variable in the pipeline for $$ROOT:
db.collection.aggregate([
{ "$group": {
"_id": "$article_id",
"docs": {
"$push": "$$ROOT"
},
"votes": { "$sum": 1 }
}},
{ "$match": { "votes": { "$gt": 3 } } },
{ "$unwind": "$docs" },
{ "$project": {
"user_id": "$docs.user_id",
"article_id": "$docs.article_id",
"score": "$docs.score"
}}
])
Otherwise you can accept that you are doing this in a few steps and process the list of "article_id" values returned with a "count" greater than three:
var ids = db.collection.aggregate([
{ "$group": {
"_id": "$article_id",
"votes": { "$sum": 1 }
}},
{ "$match": { "votes": { "$gt": 3 } } },
]).toArray().map(function(x){ return x._id });
db.collection.find({ "article_id": { "$in": ids } })
If that was a shell operation then you would use the "results" key from the array of results that was returned by default in versions earlier to 2.6.