Get a count of total documents with MongoDB when u

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

You can do this in one query. First you run a count and within that run the limit() function.

In Node.js and Express.js, you will have to use it like this to be able to use the "count" function along with the toArray's "result".

var curFind = db.collection('tasks').find({query});

Then you can run two functions after it like this (one nested in the other)

curFind.count(function (e, count) {

// Use count here

    curFind.skip(0).limit(10).toArray(function(err, result) {

    // Use result here and count here

    });

});

Answer 2

No, there is no other way. Two queries - one for count - one with limit. Or you have to use a different database. Apache Solr for instance works like you want. Every query there is limited and returns totalCount.

Answer 3

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" }
    ]
  }}
])