Get the number of documents in a MongoDB collection with unique/distinct values for a field

Some times we need to get the number of documents with a distinct/unique value. An example use case is in the classic blog example.

• Get the number of authors with at least one post.
• Get the number of distinct tags/categories

You can use the following commands to get the answers for the above questions

db.posts.distinct('author').length
db.posts.distinct('tag').length

Reference:

• Aggregation – MongoDB
• JavaScript array object