I use the following structure: class Tag (db.Expando): label = StringProperty() parent_ = SelfReferenceProperty(collection_name='children')
This is a simplified for illustration purposes, but it works well enough for us because we just grab all Tag entities and build the tree in memory with a single simple function. Of course, we will never be more than 3-4 deep and always have less than 100 tags total. On Jul 13, 5:17 am, "Nick Johnson (Google)" <nick.john...@google.com> wrote: > Hi Peter, > > On Sun, Jul 12, 2009 at 8:31 PM, Peter Cicman<pcic...@gmail.com> wrote: > > > I had spend some time today with building tree structure using > > datastore. I need a real tree, for real world, so there must be a > > possibility to move node with all descendants. There must also be a > > quick way to find all ancestors, children and descendants. > > I'm not aware of any tree structure that fits both those requirements > - in fact, I'm fairly sure it's not possible. > > There are two basic approaches to representing trees: Ones that > encompass the node's position in the entire tree, such as materialized > path and nested set notation, and ones that only encompass the node's > relationship to neighbouring nodes, such as adjacency lists. The > former permit easily querying all descendants, while the latter make > it easy to move entire branches. I don't think it's possible to have > both at once. > > If moving a branch is less common than querying, and your trees are > relatively shallow (Say, less than 100 levels deep), I would recommend > using the materialized path approach: Have a ListProperty that lists > all the ancestors of the node. > > -Nick Johnson > > > > > So, datastore keys can not be used for tree like this - because key > > can not change, and i would like to prevent node copying/deletion, > > because this must be always done on node and all its descendants which > > may end up with tons of sql queries. > > > Design i made should fit for me, but maybe there is some common > > solution for this problem... Anybody knows about some optimal > > solution? (i can not use serialization) > > > Thanks a lot! > > -- > Nick Johnson, App Engine Developer Programs Engineer > Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration > Number: 368047 --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appengine@googlegroups.com To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en -~----------~----~----~----~------~----~------~--~---
