Mongo's distinct with occurrences

mongo-distinct-with-count.js

/**
 * Like Mongo's db.collection.distinct but with counts for each field value.
 * See https://docs.mongodb.org/v3.0/reference/method/db.collection.distinct.
 * 
 * Example:
 *   > distinct('inventory', 'items.sku', { dept: 'A' })
 *   { "111" : 2, "333" : 1 }
 */
function distinct(collection, field, query) {
  var map = {}
  db[collection].aggregate([
    { $match: query || {} },
    { $group: { _id: '$' + field, c: { $sum: 1 } } },
    { $sort: { _id: -1 } },
  ]).forEach(function (g) {
    map[g._id] = g.c
  })
  return map
}