#Simple monkey patch would work

module ActiveRecord
  Base.send :attr_accessor, :_row_

  module Associations
    class JoinDependency
      JoinBase && class JoinPart
        def instantiate_with_row(row, *args)
          instantiate_without_row(row, *args).tap { |i| i._row_ = row }
        end
        alias_method_chain :instantiate, :row
      end
    end
  end
end

#Then you can get a column value from a row:

Post.includes(:comments).select("posts.*, 1 as testval").first._row_['test_val']