This article introduces sample code for calculating unique values in MongoDB using distinct and aggregate.

Sample Code for Calculating unique Values in MongoDB

Calculate unique users with distinct

Just use db.collection.distinct(‘field’) and you’re done.

Here’s a sample query to get unique users from orders:

var users = db.orders.distinct('user');

For simple conditions, using distinct is sufficient, but with a large number of documents, errors may occur.

exception: distinct too big, 16mb cap

When there are a large number of order documents that exceed the 16MB limit, the following error occurs. In this case, use aggregate for aggregation as introduced next.

> var users = db.orders.distinct('user');
2019-06-22T12:52:05.819+0000 E QUERY    Error: distinct failed: {
  "errmsg" : "exception: distinct too big, 16mb cap",
  "code" : 17217,
  "ok" : 0
}
    at Error ()
    at DBCollection.distinct (src/mongo/shell/collection.js:1237:15)
    at (shell):1:19 at src/mongo/shell/collection.js:1237

Calculate unique users with aggregate

Using aggregate won’t produce the exception: distinct too big, 16mb cap error, but the execution time for .forEach() .push() can be considerable.

var users = [];
db.orders.aggregate(
  [
    {
      $group: { _id: '$user' }
    }
  ]
).forEach(function(result) {
  users.push(result._id);
});

That’s all from the Gemba on aggregating unique values with MongoDB queries.