How to get count before limit in the Mongo aggregation pipeline

Question

In the following query:

db.orders.aggregate([{ $match : { status: "A"}, { $limit: 5} }]);

How can I get the count of documents after the $match but before the $limit is applied?

I still want to return an array of 5 documents. But if I use $group it seems that it would not preserve the array of documents. Can this be done in one call, or do I have to make two calls?

Answer 1

Starting in Mongo 3.4, the $facet aggregation stage allows processing multiple aggregation pipelines within a single stage on the same set of input documents:

// { "status" : "A", "v" : 3 }
// { "status" : "B", "v" : 14 }
// { "status" : "A", "v" : 7 }
// { "status" : "A", "v" : 5 }
db.collection.aggregate(
  { $match: { status: "A"} },
  { $facet: {
      count:  [{ $count: "count" }],
      sample: [{ $limit: 2 }]
  }}
)
// {
//   "count" : [ { "count" : 3 } ],
//   "sample" : [ { "status" : "A", "v" : 3 }, { "status" : "A", "v" : 7 } ]
// }

This:

• starts by matching documents whose status is A.

• and then produces via a $facet stage two fields that out of two distinct aggregation pipelines:

  • $count that simply provides the number of documents resulting from the preceding $match stage.

  • sample that is a $limited extract of documents resulting from the preceding $match stage.

Answer 2

Unfortunately, right now you have to make two calls, one call with the $limit operator for your results followed by a second call for the count. You could use the aggregation framework without the $limit operator and with a $group operator to calculate a count or as wdberkeley points out you can pass your criteria to .count() to get a count instead of using the aggregation framework if you are using a single match stage.

See MongoDB - Aggregation Framework (Total Count).

Answer 3

This can now be done in one call as of version 3.4 with the $facet aggregation. See here for an example: Mongo aggregation with paginated data and totals