Mongo count occurrences of each value for a set of documents

Question

I have some documents like this:

{   "user": '1' }, { "user": '1' }, {   "user": '2' }, {   "user": '3' } 

I'd like to be able to get a set of all the different users and their respective counts, sorted in decreasing order. So my output would be something like this:

{   '1': 2,   '2': 1,   '3': 1 } 

I think this can be done with a Mongo aggregate(), but I'm having a lot of trouble figuring out the right flow for this.

Answer 1

You can get result (not in your required format) via aggregation

db.collection.aggregate(    {$group : { _id : '$user', count : {$sum : 1}}} ).result 

the output for your sample documents is:

"0" : {     "_id" : "2",     "count" : 1 }, "1" : {     "_id" : "3",     "count" : 1 }, "2" : {     "_id" : "1",     "count" : 2 } 

Answer 2

For anyone reading this in Jan 2019 the accepted answer does not currently work in Robo3T (returns a pipeline.length - 1 error).

You must:

a) wrap the query in a set of square brackets []

b) remove .result from the end

https://github.com/Studio3T/robomongo/issues/1519#issuecomment-441348191

Here's an update to the accepted answer by @disposer that works for me in Robo3T.

db.getCollection('collectionName').aggregate(     [ {$group : { _id : '$user', count : {$sum : 1}}} ] )