title: mongodb count num of distinct values per field/key
body: <p>Is there a query for calculating how many distinct values a field contains in DB.</p>
  
  <p>f.e I have a field for country and there are 8 types of country values (spain, england, france, etc...)</p>
  
  <p>If someone adds more documents with a new country I would like the query to return 9.</p>
  
  <p>Is there easier way then group and count?</p>