MongoDB - Aggregation Framework (Total Count)

Question

When running a normal "find" query on MongoDB I can get the total result count (regardless of limit) by running "count" on the returned cursor. So, even if I limit to result set to 10 (for example) I can still know that the total number of results was 53 (again, for example).

If I understand it correctly, the aggregation framework, however, doesn't return a cursor but simply the results. And so, if I used the $limit pipeline operator, how can I know the total number of results regardless of said limit?

I guess I could run the aggregation twice (once to count the results via $group, and once with $limit for the actual limited results), but this seems inefficient.

An alternative approach could be to attach the total number of results to the documents (via $group) prior to the $limit operation, but this also seems inefficient as this number will be attached to every document (instead of just returned once for the set).

Am I missing something here? Any ideas? Thanks!

For example, if this is the query:

db.article.aggregate(
    { $group : {
        _id : "$author",
        posts : { $sum : 1 }
    }},
    { $sort : { posts: -1 } },
    { $limit : 5 }
);

How would I know how many results are available (before $limit)? The result isn't a cursor, so I can't just run count on it.

Answer 1

$facets aggregation operation can be used for Mongo versions >= 3.4. This allows to fork at a particular stage of a pipeline in multiple sub-pipelines allowing in this case to build one sub pipeline to count the number of documents and another one for sorting, skipping, limiting.

This allows to avoid making same stages multiple times in multiple requests.