How to find records that have duplicate data using Active Record

Question

What is the best way to find records with duplicate values in a column using ruby and the new Activerecord?

Answer 1

Here's how I solved it with the AREL helpers, and no custom SQL:

Person.select("COUNT(last_name) as total, last_name")
  .group(:last_name)
  .having("COUNT(last_name) > 1")
  .order(:last_name)
  .map{|p| {p.last_name => p.total} }

Really, it's just a nicer way to write the SQL. This finds all records that have duplicate last_name values, and tells you how many and what the last names are in a nice hash.