instance method arel_columns

# File activerecord/lib/active_record/relation/query_methods.rb, line 1642
      def arel_columns(columns)
        columns.flat_map do |field|
          case field
          when Symbol
            arel_column(field.to_s) do |attr_name|
              adapter_class.quote_table_name(attr_name)
            end
          when String
            arel_column(field, &:itself)
          when Proc
            field.call
          when Hash
            arel_columns_from_hash(field)
          else
            field
          end
        end
      end