snippet.rb

module FindDuplicates
  # Returns a hash of duplicate values and their counts, select can be 
  # anything that evaluates to a single SQL column.
  # Warning: select and minimum_count are not SQL escaped!
  #
  #  User.find_duplicates(:username)        # { 'jqr' => 2 }
  #  User.find_duplicates(LOWER(city), 5)   # { 'indianaplis' => 25, 'new york' => 201 }
  def find_duplicates(select, minimum_count = 2)
    rows = connection.select_all("SELECT #{select} AS value, COUNT(*) AS count FROM #{table_name} GROUP BY #{select} HAVING COUNT(*) >= #{minimum_count}")
    rows.inject({}) do |acc, row|
      acc[row['value']] = row['count'].to_i
      acc
    end
  end
end