mongodb count num of distinct values per field/key

Question

Is there a query for calculating how many distinct values a field contains in DB.

f.e I have a field for country and there are 8 types of country values (spain, england, france, etc...)

If someone adds more documents with a new country I would like the query to return 9.

Is there easier way then group and count?

Answer 1

MongoDB has a distinct command which returns an array of distinct values for a field; you can check the length of the array for a count.

There is a shell db.collection.distinct() helper as well:

> db.countries.distinct('country');
[ "Spain", "England", "France", "Australia" ]

> db.countries.distinct('country').length
4

As noted in the MongoDB documentation:

Results must not be larger than the maximum BSON size (16MB). If your results exceed the maximum BSON size, use the aggregation pipeline to retrieve distinct values using the $group operator, as described in Retrieve Distinct Values with the Aggregation Pipeline.