Get a count of total documents with MongoDB when using limit

Question

I am interested in optimizing a "pagination" solution I'm working on with MongoDB. My problem is straight forward. I usually limit the number of documents returned using the limit() functionality. This forces me to issue a redundant query without the limit() function in order for me to also capture the total number of documents in the query so I can pass to that to the client letting them know they'll have to issue an additional request(s) to retrieve the rest of the documents.

Is there a way to condense this into 1 query? Get the total number of documents but at the same time only retrieve a subset using limit()? Is there a different way to think about this problem than I am approaching it?

Answer 1

Mongodb 3.4 has introduced $facet aggregation

which processes multiple aggregation pipelines within a single stage on the same set of input documents.

Using $facet and $group you can find documents with $limit and can get total count.

You can use below aggregation in mongodb 3.4

db.collection.aggregate([
  { "$facet": {
    "totalData": [
      { "$match": { }},
      { "$skip": 10 },
      { "$limit": 10 }
    ],
    "totalCount": [
      { "$group": {
        "_id": null,
        "count": { "$sum": 1 }
      }}
    ]
  }}
])

Even you can use $count aggregation which has been introduced in mongodb 3.6.

You can use below aggregation in mongodb 3.6

db.collection.aggregate([
  { "$facet": {
    "totalData": [
      { "$match": { }},
      { "$skip": 10 },
      { "$limit": 10 }
    ],
    "totalCount": [
      { "$count": "count" }
    ]
  }}
])