columns_for_distinct

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 473
        def columns_for_distinct(columns, orders) #:nodoc:
          order_columns = orders.map{ |s|
              # Convert Arel node to string
              s = s.to_sql unless s.is_a?(String)
              # Remove any ASC/DESC modifiers
              s.gsub(/\s+(ASC|DESC)\s*(NULLS\s+(FIRST|LAST)\s*)?/, '')
            }.reject(&:blank?).map.with_index { |column, i| "#{column} AS alias_#{i}" }

          [super, *order_columns].join(', ')
        end