How can I count unique values in a table in Rails?

Question

I have a table "stock" which consists of many package_ids

package_id = 1  
package_id = 3  
package_id = 2  
package_id = 3  
package_id = 3  
package_id = 4  
package_id = 2

What is the most elegant way to:

• count each unique package_id in the db, e.g.: package_id 1 = one time in the db; package_id 2 = two times in the db; package_id 3 = three times in the DB ...
• echo the top 3 of package IDs afterwards

I have tried this step by step:

• counting each single package_id (Stock.where(:package_id => 1).count)
• putting that all in an array
• and sort that array from high to low (only first 3 items)

This however does not seems to be an effective path though.

Answer 1

How about:

Stock.group(:package_id).count

It will return a hash having package_id as a key and the count as a value:

{ package_id1: count1, package_id2: count2 ....}

Answer 2

Try with this to get the 'Top 3':

Stock.select('package_id, count(*) as c').group(:package_id).order('c DESC').limit(3)