Hi Nick, hi Vince, i did come up with something like this yesterday:
class Tree(db.Expando):
parents = ListProperty()
level = IntegerProperty()
position = IntegerProperty()
properties:
parents: sorted list of all parent keys, sorting order top/down,
so parents[-1] gives object direct parent
level: nested level, for root nodes = 0, for children of root node
= 1, etc..
position: node position in current subtree, 0 on top, n on bottom
Probably it will work, parent, children, descendants selection should
be possible. Its probably very close to solution described by Nick.
----
But anyway, the key paths looks like so nice solution for tree
structure, so i'm still thinking about them. It brings up some
questions:
- is there a possibility to use other property for path than a
key? probably not if i understand the implementation
- moving the node with all descendants is currently be possible
this way: copy node, change key, save it, delete original node. the
question is - if the datastore is represented like a hash, why key
change isn't allowed? What is so difficult on this?
- if the key change really isn't possible on datastore level,
can't there be some helper in datastore API for doing this (change
instance key / change instance keys on node + all descendants,
eventually also delete method, which performs descendants deletion
when parent gets removed)..
Thanks...
On Jul 13, 1:51 pm, Vince Stross <[email protected]> wrote:
> 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)" <[email protected]>
> wrote:
>
>
>
> > Hi Peter,
>
> > On Sun, Jul 12, 2009 at 8:31 PM, Peter Cicman<[email protected]> 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
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