Counting Documents

The template API offers various methods to count the number of documents matching a given criteria. One of them outlined below.

template.query(Person.class)
    .matching(query(where("firstname").is("luke")))
    .count();

In pre-3.x versions of SpringData MongoDB the count operation used MongoDBs internal collection statistics. With the introduction of MongoDB Transactions this was no longer possible because statistics would not correctly reflect potential changes during a transaction requiring an aggregation-based count approach. So in version 2.x MongoOperations.count() would use the collection statistics if no transaction was in progress, and the aggregation variant if so.

As of Spring Data MongoDB 3.x any count operation uses regardless the existence of filter criteria the aggregation-based count approach via MongoDBs countDocuments. If the application is fine with the limitations of working upon collection statistics MongoOperations.estimatedCount() offers an alternative.

By setting MongoTemplate#useEstimatedCount(…) to true MongoTemplate#count(…) operations, that use an empty filter query, will be delegated to estimatedCount, as long as there is no transaction active and the template is not bound to a session. It will still be possible to obtain exact numbers via MongoTemplate#exactCount, but may speed up things.

MongoDBs native countDocuments method and the $match aggregation, do not support $near and $nearSphere but require $geoWithin along with $center or $centerSphere which does not support $minDistance (see jira.mongodb.org/browse/SERVER-37043).

Therefore a given Query will be rewritten for count operations using Reactive-/MongoTemplate to bypass the issue like shown below.

{ location : { $near : [-73.99171, 40.738868], $maxDistance : 1.1 } } (1)
{ location : { $geoWithin : { $center: [ [-73.99171, 40.738868], 1.1] } } } (2)

{ location : { $near : [-73.99171, 40.738868], $minDistance : 0.1, $maxDistance : 1.1 } } (3)
{$and :[ { $nor :[ { location :{ $geoWithin :{ $center :[ [-73.99171, 40.738868 ], 0.01] } } } ]}, { location :{ $geoWithin :{ $center :[ [-73.99171, 40.738868 ], 1.1] } } } ] } (4)

1	Count source query using `$near`.
2	Rewritten query now using `$geoWithin` with `$center`.
3	Count source query using `$near` with `$minDistance` and `$maxDistance`.
4	Rewritten query now a combination of `$nor` `$geowithin` critierias to work around unsupported `$minDistance`.