arel_columns

# File activerecord/lib/active_record/relation/query_methods.rb, line 1052
      def arel_columns(columns)
        columns.flat_map do |field|
          case field
          when Symbol
            field = field.to_s
            arel_column(field) { connection.quote_table_name(field) }
          when String
            arel_column(field) { field }
          when Proc
            field.call
          else
            field
          end
        end
      end