blog 1
    - reply 2
      - reply 4
      - reply 5
        - reply 6
    - reply 3
      - reply 7


##</code><code>##


      ancestor_id    descendant_id
      ----------------------------
          1              2
          1              3
          1              4
          1              5
          1              6
          1              7
          2              4
          2              5
          2              6
          3              7
          5              6