How to count occurence of each value in array? - arrays

Question

I have a database of ISSUES in MongoDB, some of the issues have comments, which is an array; each comments has a writer. How can I count the number of comments each writer has written?
I've tried
db.test.issues.group(
{
key = "comments.username":true;
initial: {sum:0},
reduce: function(doc, prev) {prev.sum +=1},
}
);
but no luck :(
A Sample:
{
"_id" : ObjectId("50f48c179b04562c3ce2ce73"),
"project" : "Ruby Driver",
"key" : "RUBY-505",
"title" : "GETMORE is sent to wrong server if an intervening query unpins the connection",
"description" : "I've opened a pull request with a failing test case demonstrating the bug here: https://github.com/mongodb/mongo-ruby-driver/pull/134\nExcerpting that commit message, the issue is: If we do a secondary read that is large enough to require sending a GETMORE, and then do another query before the GETMORE, the secondary connection gets unpinned, and the GETMORE gets sent to the wrong server, resulting in CURSOR_NOT_FOUND, even though the cursor still exis ts on the server that was initially queried.",
"status" : "Open",
"components" : [
"Replica Set"
],
"affected_versions" : [
"1.7.0"
],
"type" : "Bug",
"reporter" : "Nelson Elhage",
"priority" : "major",
"assignee" : "Tyler Brock",
"resolution" : "Unresolved",
"reported_on" : ISODate("2012-11-17T20:30:00Z"),
"votes" : 3,
"comments" : [
{
"username" : "Nelson Elhage",
"date" : ISODate("2012-11-17T20:30:00Z"),
"body" : "Thinking some more"
},
{
"username" : "Brandon Black",
"date" : ISODate("2012-11-18T20:30:00Z"),
"body" : "Adding some findings of mine to this ticket."
},
{
"username" : "Nelson Elhage",
"date" : ISODate("2012-11-18T20:30:00Z"),
"body" : "I think I tracked down the 1.9 dependency."
},
{
"username" : "Nelson Elhage",
"date" : ISODate("2012-11-18T20:30:00Z"),
"body" : "Forgot to include a link"
}
]
}

Answer 1

You forgot the curly braces on the key value and you need to terminate that line with a , instead of a ;.
db.issues.group({
key: {"comments.username":true},
initial: {sum:0},
reduce: function(doc, prev) {prev.sum +=1},
});
UPDATE
After realizing comments is an array...you'd need to use aggregate for that so that you can 'unwind' comments and then group on it:
db.issues.aggregate(
{$unwind: '$comments'},
{$group: {_id: '$comments.username', sum: {$sum: 1}}}
);
For the sample doc in the question, this outputs:
{
"result": [
{
"_id": "Brandon Black",
"sum": 1
},
{
"_id": "Nelson Elhage",
"sum": 3
}
],
"ok": 1
}

Answer 2

Just a snide answer here to compliment #JohnnyHKs answer: it sounds like your new to MongoDB and as such possibly working on a new version of MongoDB if that is the case (if not I would upgrade) either way the old group count is kinda bad. It won't, for one, work with sharding.
Instead in MongoDB 2.2 you can just do:
db.col.aggregate({$group: {_id: "$comments.username", count: {$sum: 1}}})
Or something similar. You can read more about it here: http://docs.mongodb.org/manual/applications/aggregation/