MongoDB Aggregation: How to get total records count?

Question

Since v.3.4 (i think) MongoDB has now a new aggregation pipeline operator named 'facet' which in their own words:

Processes multiple aggregation pipelines within a single stage on the same set of input documents. Each sub-pipeline has its own field in the output document where its results are stored as an array of documents.

In this particular case, this means that one can do something like this:

$result = $collection->aggregate([
  { ...execute queries, group, sort... },
  { ...execute queries, group, sort... },
  { ...execute queries, group, sort... },
  {
    $facet: {
      paginatedResults: [{ $skip: skipPage }, { $limit: perPage }],
      totalCount: [
        {
          $count: 'count'
        }
      ]
    }
  }
]);

The result will be (with for ex 100 total results):

[
  {
    "paginatedResults":[{...},{...},{...}, ...],
    "totalCount":[{"count":100}]
  }
]

This is one of the most commonly asked question to obtain the paginated result and the total number of results simultaneously in single query. I can't explain how I felt when I finally achieved it LOL.

$result = $collection->aggregate(array(
  array('$match' => $document),
  array('$group' => array('_id' => '$book_id', 'date' => array('$max' => '$book_viewed'),  'views' => array('$sum' => 1))),
  array('$sort' => $sort),

// get total, AND preserve the results
  array('$group' => array('_id' => null, 'total' => array( '$sum' => 1 ), 'results' => array( '$push' => '$$ROOT' ) ),
// apply limit and offset
  array('$project' => array( 'total' => 1, 'results' => array( '$slice' => array( '$results', $skip, $length ) ) ) )
))

Result will look something like this:

[
  {
    "_id": null,
    "total": ...,
    "results": [
      {...},
      {...},
      {...},
    ]
  }
]

Use this to find total count in resulting collection.

db.collection.aggregate( [
{ $match : { score : { $gt : 70, $lte : 90 } } },
{ $group: { _id: null, count: { $sum: 1 } } }
] );

You can use toArray function and then get its length for total records count.

db.CollectionName.aggregate([....]).toArray().length