get total of sub documents in a collection

Question

How do I get the total comments in the collection if my collection looks like this. (not the total comments per post but total for the collection.)

{
    _id: 1,
    post: 'content',
    comments: [
        {
            name: '',
            comment: ''
        }
    ]
}

If I have post A with 3 comments and post B with 5 comments. The result should be 8.

Answer 1

You could use the aggregation framework:

> db.prabir.aggregate(
    { $unwind : "$comments" },
    { $group: {
        _id: '',
        count: { $sum: 1 }
    }
})
{ "result" : [ { "_id" : "", "count" : 8 } ], "ok" : 1 }

In a nutshell this (temporarily) creates a separate document for each comment and then increments count for each document.

---

For a large number of posts and comments it might be more efficient to keep track of the number of comments. When ever a comment is added you also increment a counter. Example:

// Insert a comment
> comment = { name: 'JohnDoe', comment: 'FooBar' }
> db.prabir.update(
    { post: "A" },
    {
        $push: { comments: comment },
        $inc: { numComments: 1 }
    }
)

Using the aggregation framework again:

> db.prabir.aggregate(
    { $project : { _id: 0, numComments: 1 }},
    { $group: {
        _id: '',
        count: { $sum: "$numComments" }
    }
})
{ "result" : [ { "_id" : "", "count" : 8 } ], "ok" : 1 }