merge_select_values

# File activerecord/lib/active_record/relation/merger.rb, line 84
        def merge_select_values
          return if other.select_values.empty?

          if other.klass == relation.klass
            relation.select_values |= other.select_values
          else
            relation.select_values |= other.instance_eval do
              arel_columns(select_values)
            end
          end
        end