Well the last ORM tip seemed to get a bit of traffic so I thought I would throw another one together. This time it covers the little known ORM_Tree library. This is (IMO) a spectacular example of the flexibility and power of Kohana, by taking a look at this ORM extension (system/libraries/ORM_Tree) you can see quite how easily this age old storage pattern is solved by extending the ORM class.
Using it is even simpler!